Speed up PostingsEnum when reading positions. (apache#14032)

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.

lucene/CHANGES.txt

@@ -119,6 +119,9 @@ Optimizations
 * GITHUB#14023: Make JVM inlining decisions more predictable in our main
   queries. (Adrien Grand)
 
+* GITHUB#14032: Speed up PostingsEnum when positions are requested.
+  (Adrien Grand)
+
 Bug Fixes
 ---------------------
 * GITHUB#13832: Fixed an issue where the DefaultPassageFormatter.format method did not format passages as intended