Speed up PostingsEnum when reading positions.

[jpountz]

This PR changes the following:

• As much work as possible is moved from nextDoc()/advance() to nextPosition(). This helps only pay the overhead of reading positions when all query terms agree on a candidate.
• Frequencies are read lazily. Again, this helps in case a document is needed in a block, but clauses do not agree on a common candidate match, so frequencies are never decoded.
• A few other minor optimizations.

[jpountz]

                        TaskQPS baseline      StdDevQPS my_modified_version      StdDev                Pct diff p-value
                      IntNRQ      110.78     (11.4%)      108.66     (13.0%)   -1.9% ( -23% -   25%) 0.739
                 CountPhrase        4.37      (1.6%)        4.30      (1.2%)   -1.7% (  -4% -    1%) 0.009
                      OrMany       20.20      (3.6%)       19.88      (4.2%)   -1.6% (  -9% -    6%) 0.381
             DismaxOrHighMed       85.25      (1.7%)       84.24      (3.9%)   -1.2% (  -6% -    4%) 0.403
            DismaxOrHighHigh       69.09      (2.0%)       68.36      (3.6%)   -1.1% (  -6% -    4%) 0.435
                    SpanNear        2.17     (12.4%)        2.14     (12.0%)   -1.0% ( -22% -   26%) 0.855
                   CountTerm     8796.37      (5.0%)     8709.80      (2.8%)   -1.0% (  -8% -    7%) 0.605
                 OrStopWords       34.15      (8.2%)       33.89      (5.8%)   -0.8% ( -13% -   14%) 0.820
           FilteredAnd3Terms      196.11      (2.3%)      194.82      (1.9%)   -0.7% (  -4% -    3%) 0.503
                    Or3Terms      175.20      (5.2%)      174.24      (3.8%)   -0.6% (  -9% -    8%) 0.797
              FilteredOrMany       16.97      (5.4%)       16.88      (5.0%)   -0.5% ( -10% -   10%) 0.834
          Or2Terms2StopWords      165.63      (4.6%)      164.83      (3.5%)   -0.5% (  -8% -    7%) 0.801
                      Fuzzy1       81.21      (1.6%)       80.83      (1.5%)   -0.5% (  -3% -    2%) 0.529
                SloppyPhrase        1.78     (10.5%)        1.77      (3.6%)   -0.4% ( -13% -   15%) 0.907
 FilteredAnd2Terms2StopWords      201.36      (1.9%)      200.55      (1.3%)   -0.4% (  -3% -    2%) 0.599
          FilteredAndHighMed      130.30      (2.7%)      129.81      (2.6%)   -0.4% (  -5% -    5%) 0.764
                      Fuzzy2       76.58      (1.4%)       76.29      (1.2%)   -0.4% (  -2% -    2%) 0.536
                FilteredTerm      156.92      (2.5%)      156.39      (2.1%)   -0.3% (  -4% -    4%) 0.757
                   And3Terms      179.52      (3.2%)      179.00      (3.5%)   -0.3% (  -6% -    6%) 0.855
         FilteredAndHighHigh       63.65      (2.9%)       63.47      (2.0%)   -0.3% (  -5% -    4%) 0.813
        FilteredAndStopWords       48.99      (2.9%)       48.86      (1.9%)   -0.3% (  -4% -    4%) 0.828
         And2Terms2StopWords      167.29      (3.1%)      166.94      (3.3%)   -0.2% (  -6% -    6%) 0.887
           TermDayOfYearSort      631.10      (1.3%)      630.31      (1.2%)   -0.1% (  -2% -    2%) 0.835
            FilteredOr3Terms      168.75      (1.2%)      168.59      (1.4%)   -0.1% (  -2% -    2%) 0.879
             CountAndHighMed      169.49      (1.2%)      169.42      (1.3%)   -0.0% (  -2% -    2%) 0.948
  FilteredOr2Terms2StopWords      151.07      (1.0%)      151.02      (1.2%)   -0.0% (  -2% -    2%) 0.948
                AndStopWords       32.08      (5.1%)       32.07      (5.0%)   -0.0% (  -9% -   10%) 0.993
           FilteredOrHighMed      157.50      (1.1%)      157.49      (0.6%)   -0.0% (  -1% -    1%) 0.992
            CountAndHighHigh       57.71      (1.1%)       57.71      (0.8%)    0.0% (  -1% -    1%) 1.000
                    PKLookup      282.75      (1.9%)      282.86      (1.4%)    0.0% (  -3% -    3%) 0.958
                     Respell       54.63      (1.1%)       54.70      (1.5%)    0.1% (  -2% -    2%) 0.843
                TermGroup100       24.50      (4.5%)       24.54      (3.8%)    0.2% (  -7% -    8%) 0.929
              CountOrHighMed      141.72      (1.9%)      142.10      (1.1%)    0.3% (  -2% -    3%) 0.717
         FilteredOrStopWords       44.42      (1.9%)       44.56      (2.3%)    0.3% (  -3% -    4%) 0.759
             AndHighOrMedMed       43.17      (2.5%)       43.31      (0.7%)    0.3% (  -2% -    3%) 0.720
             CountOrHighHigh       74.88      (1.6%)       75.12      (1.3%)    0.3% (  -2% -    3%) 0.653
           CombinedOrHighMed       70.87      (5.8%)       71.13      (3.7%)    0.4% (  -8% -   10%) 0.873
               TermMonthSort     3404.71      (2.0%)     3418.15      (2.6%)    0.4% (  -4% -    5%) 0.718
          CombinedOrHighHigh       18.54      (5.9%)       18.64      (3.7%)    0.5% (  -8% -   10%) 0.821
               TermTitleSort      159.32      (3.5%)      160.30      (2.9%)    0.6% (  -5% -    7%) 0.686
                   OrHighMed      196.12      (5.6%)      197.40      (4.9%)    0.7% (  -9% -   11%) 0.794
            IntervalsOrdered        2.37      (2.7%)        2.38      (3.2%)    0.7% (  -5% -    6%) 0.627
          FilteredOrHighHigh       65.75      (1.7%)       66.20      (1.5%)    0.7% (  -2% -    3%) 0.365
                TermGroup10K       20.11      (3.6%)       20.25      (3.5%)    0.7% (  -6% -    8%) 0.682
                  DismaxTerm      605.77      (2.9%)      610.10      (2.8%)    0.7% (  -4% -    6%) 0.598
                 TermGroup1M       19.82      (3.1%)       19.97      (3.5%)    0.8% (  -5% -    7%) 0.616
            AndMedOrHighHigh       59.49      (1.9%)       59.96      (1.6%)    0.8% (  -2% -    4%) 0.351
                  TermDTSort      274.15      (1.8%)      276.92      (2.3%)    1.0% (  -3% -    5%) 0.309
                    Wildcard       73.73      (4.2%)       74.49      (3.0%)    1.0% (  -5% -    8%) 0.545
          CombinedAndHighMed       54.61      (4.7%)       55.18      (2.5%)    1.0% (  -5% -    8%) 0.558
                  AndHighMed      131.04      (2.8%)      132.48      (1.7%)    1.1% (  -3% -    5%) 0.308
                  OrHighHigh       52.99      (7.0%)       53.63      (6.2%)    1.2% ( -11% -   15%) 0.697
                        Term      470.85      (4.2%)      476.71      (4.3%)    1.2% (  -7% -   10%) 0.537
         CombinedAndHighHigh       14.94      (5.0%)       15.13      (2.5%)    1.2% (  -5% -    9%) 0.505
                  OrHighRare      271.47      (8.0%)      275.00      (7.1%)    1.3% ( -12% -   17%) 0.715
                CombinedTerm       29.64      (5.1%)       30.08      (3.8%)    1.5% (  -7% -   10%) 0.480
                TermBGroup1M       24.87      (3.9%)       25.26      (3.8%)    1.6% (  -5% -    9%) 0.386
                 AndHighHigh       44.95      (3.0%)       45.68      (1.7%)    1.6% (  -3% -    6%) 0.158
                     Prefix3      127.48      (6.7%)      129.68      (7.8%)    1.7% ( -11% -   17%) 0.615
              TermBGroup1M1P       36.78      (4.9%)       37.59      (5.4%)    2.2% (  -7% -   13%) 0.364
                      Phrase       13.78      (2.7%)       14.90      (2.8%)    8.1% (   2% -   13%) 0.000
              FilteredPhrase       26.01      (3.4%)       30.92      (1.6%)   18.9% (  13% -   24%) 0.000

[rmuir]

great to see the frequencies finally lazy-decoded

[jpountz]

FilteredPhrase had a good 25% speedup, Phrase had a good 11% speedup, but other phrase queries had a slowdown, e.g. SloppyPhrase (-8%) and CountPhrase (-7%).