[OPTIMIZATION] Explicitly make xidmap shards as nil

[ashish-goswami]

In bulk/live loader, we use xidmap to keep mapping of external ids to UUIDs. In case of bulk
loader, this is required only in map phase. Hence at the completion of map phase we make
xidmap instance used in bulk loader as nil. Ideally this should be good for xidmap GC and
release of underlying memory. But this is not happening and xidmap still shows up in memory
profiles even during reduce phase.

This is fixed by making xidmap.shards = nil in xidmap.Flush() method.

Bulk loader memory profile on 21M dataset during reduce phase:
Profile has been taken after reduce phase is completed 50%.
Master

(pprof) top20
Showing nodes accounting for 448.41MB, 98.03% of 457.44MB total
Dropped 60 nodes (cum <= 2.29MB)
Showing top 20 nodes out of 44
      flat  flat%   sum%        cum   cum%
     221MB 48.31% 48.31%   222.50MB 48.64%  github.com/dgraph-io/dgraph/xidmap.(*XidMap).AssignUid
   83.20MB 18.19% 66.50%    83.20MB 18.19%  github.com/dgraph-io/badger/v2/skl.newArena
   65.50MB 14.32% 80.82%    65.50MB 14.32%  strings.(*Builder).WriteString
   38.16MB  8.34% 89.16%    38.16MB  8.34%  bytes.makeSlice
    8.67MB  1.90% 91.06%     8.67MB  1.90%  github.com/dgraph-io/badger/v2/table.(*Builder).addHelper
    7.42MB  1.62% 92.68%    10.42MB  2.28%  github.com/dgraph-io/badger/v2/pb.(*TableIndex).Unmarshal
    6.59MB  1.44% 94.12%     6.59MB  1.44%  github.com/dgraph-io/dgraph/protos/pb.(*PostingList).Marshal
    6.04MB  1.32% 95.44%     6.04MB  1.32%  github.com/dgraph-io/ristretto/z.(*Bloom).Size
       5MB  1.09% 96.53%        5MB  1.09%  github.com/dgraph-io/dgraph/dgraph/cmd/bulk.(*mapIterator).Next
       3MB  0.66% 97.19%        3MB  0.66%  github.com/dgraph-io/badger/v2/pb.(*BlockOffset).Unmarshal
    1.22MB  0.27% 97.46%     6.22MB  1.36%  github.com/dgraph-io/dgraph/dgraph/cmd/bulk.(*reducer).reduce
    1.11MB  0.24% 97.70%     8.20MB  1.79%  github.com/dgraph-io/dgraph/dgraph/cmd/bulk.(*reducer).toList
       1MB  0.22% 97.92%    33.51MB  7.33%  github.com/dgraph-io/badger/v2/table.(*Builder).finishBlock
    0.50MB  0.11% 98.03%     6.65MB  1.45%  github.com/dgraph-io/badger/v2.(*StreamWriter).Write
         0     0% 98.03%    38.16MB  8.34%  bytes.(*Buffer).Write
         0     0% 98.03%    38.16MB  8.34%  bytes.(*Buffer).grow
         0     0% 98.03%    83.70MB 18.30%  github.com/dgraph-io/badger/v2.(*DB).dropAll
         0     0% 98.03%    83.70MB 18.30%  github.com/dgraph-io/badger/v2.(*StreamWriter).Prepare
         0     0% 98.03%     6.15MB  1.35%  github.com/dgraph-io/badger/v2.(*logFile).encodeEntry
         0     0% 98.03%    42.18MB  9.22%  github.com/dgraph-io/badger/v2.(*sortedWriter).Add

This PR:

(pprof) top20
Showing nodes accounting for 154.94MB, 98.96% of 156.57MB total
Dropped 18 nodes (cum <= 0.78MB)
Showing top 20 nodes out of 46
      flat  flat%   sum%        cum   cum%
   83.20MB 53.14% 53.14%    83.20MB 53.14%  github.com/dgraph-io/badger/v2/skl.newArena
   36.53MB 23.33% 76.47%    36.53MB 23.33%  bytes.makeSlice
    8.67MB  5.54% 82.01%     8.67MB  5.54%  github.com/dgraph-io/badger/v2/table.(*Builder).addHelper
    6.04MB  3.86% 85.87%     6.04MB  3.86%  github.com/dgraph-io/ristretto/z.(*Bloom).Size
    5.71MB  3.65% 89.52%     8.71MB  5.56%  github.com/dgraph-io/badger/v2/pb.(*TableIndex).Unmarshal
    5.60MB  3.57% 93.09%     5.60MB  3.57%  github.com/dgraph-io/dgraph/protos/pb.(*PostingList).Marshal
       3MB  1.92% 95.01%        3MB  1.92%  github.com/dgraph-io/badger/v2/pb.(*BlockOffset).Unmarshal
    2.03MB  1.30% 96.31%     2.03MB  1.30%  bufio.NewReaderSize
    1.50MB  0.96% 97.26%        2MB  1.28%  github.com/dgraph-io/dgraph/dgraph/cmd/bulk.(*mapIterator).Next
    1.07MB  0.68% 97.95%     1.07MB  0.68%  github.com/blevesearch/bleve/geo.init.0
    1.03MB  0.66% 98.61%     1.03MB  0.66%  compress/flate.(*dictDecoder).init
    0.54MB  0.35% 98.96%     2.54MB  1.63%  github.com/dgraph-io/dgraph/dgraph/cmd/bulk.(*reducer).reduce
         0     0% 98.96%    36.53MB 23.33%  bytes.(*Buffer).Write
         0     0% 98.96%    36.53MB 23.33%  bytes.(*Buffer).grow
         0     0% 98.96%     1.03MB  0.66%  compress/flate.NewReader
         0     0% 98.96%     1.03MB  0.66%  compress/gzip.(*Reader).Reset
         0     0% 98.96%     1.03MB  0.66%  compress/gzip.(*Reader).readHeader
         0     0% 98.96%     1.03MB  0.66%  compress/gzip.NewReader
         0     0% 98.96%    83.20MB 53.14%  github.com/dgraph-io/badger/v2.(*DB).dropAll
         0     0% 98.96%    83.20MB 53.14%  github.com/dgraph-io/badger/v2.(*StreamWriter).Prepare

I also ran bulk loader on freebase dataset.
System Conf: ubuntu18, 16 core CPU, 64 GB RAM
Datasize: compressed 31GB, ~3.13B RDF

Master:
Time for completion: could not complete, crashed in reduce phase after running for ~2min.
Map phase time: ~1h53m
Reduce phase time:
Peak Memory usage(RES):

This PR:
Time for completion: ~03h40m (successful completion)
Map phase time: ~1h55m
Reduce phase time: ~1h45m
Peak Memory usage(RES): ~56 GB

---

This change is 

[pawanrawal]

Reviewed 1 of 1 files at r1.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @animesh2049 and @manishrjain)

[animesh2049]

Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @animesh2049 and @manishrjain)

[manishrjain]

Reviewed 1 of 1 files at r1.
Reviewable status: complete! all files reviewed, all discussions resolved