Use sync.Pool for MapEntries in bulk loader (#3763)

This PR has a significant impact on the allocated objects and space. For addMapEntry it went down from 7.35GB to 5.48GB. For addIndexMapEntries, it went down from 15.67GB to 10.11GB. The profiles were taken 30s into bulk loader. The other allocations line up nicely to verify that (lexer.Emit and rdfChunker.Parse).

Current master:
```
Showing nodes accounting for 45.97GB, 82.78% of 55.53GB total
Dropped 112 nodes (cum <= 0.28GB)
Showing top 10 nodes out of 54
      flat  flat%   sum%        cum   cum%
   13.61GB 24.51% 24.51%    13.61GB 24.51%  github.com/dgraph-io/dgraph/lex.(*Lexer).Emit
    7.35GB 13.23% 37.74%     7.35GB 13.23%  github.com/dgraph-io/dgraph/dgraph/cmd/bulk.(*mapper).addMapEntry
    6.82GB 12.29% 50.02%    15.67GB 28.21%  github.com/dgraph-io/dgraph/dgraph/cmd/bulk.(*mapper).addIndexMapEntries
    3.62GB  6.51% 56.54%    24.84GB 44.74%  github.com/dgraph-io/dgraph/chunker.rdfChunker.Parse
    3.04GB  5.48% 62.02%     3.04GB  5.48%  github.com/dgraph-io/dgraph/posting.NewPosting
    2.95GB  5.32% 67.34%     2.95GB  5.32%  bytes.makeSlice
    2.80GB  5.04% 72.38%     2.80GB  5.04%  github.com/dgraph-io/dgraph/gql.NQuad.createEdgePrototype
    2.71GB  4.87% 77.25%     2.71GB  4.87%  github.com/dgraph-io/dgraph/lex.NewLexer
    1.62GB  2.92% 80.18%     1.62GB  2.92%  strings.makeCutsetFunc
    1.45GB  2.61% 82.78%     1.45GB  2.61%  github.com/dgraph-io/dgraph/x.generateKey
```

This PR:
```
Showing nodes accounting for 39.94GB, 80.85% of 49.40GB total
Dropped 148 nodes (cum <= 0.25GB)
Showing top 10 nodes out of 54
      flat  flat%   sum%        cum   cum%
   13.47GB 27.26% 27.26%    13.47GB 27.26%  github.com/dgraph-io/dgraph/lex.(*Lexer).Emit
    4.40GB  8.90% 36.16%    10.11GB 20.47%  github.com/dgraph-io/dgraph/dgraph/cmd/bulk.(*mapper).addIndexMapEntries
    3.63GB  7.36% 43.52%    24.81GB 50.23%  github.com/dgraph-io/dgraph/chunker.rdfChunker.Parse
    3.47GB  7.03% 50.54%     3.47GB  7.03%  github.com/dgraph-io/dgraph/dgraph/cmd/bulk.newMapper.func1
    2.97GB  6.01% 56.55%     2.97GB  6.01%  bytes.makeSlice
    2.92GB  5.91% 62.46%     2.92GB  5.91%  github.com/dgraph-io/dgraph/posting.NewPosting
    2.76GB  5.59% 68.05%     2.76GB  5.59%  github.com/dgraph-io/dgraph/lex.NewLexer
    2.69GB  5.45% 73.51%     2.69GB  5.45%  github.com/dgraph-io/dgraph/gql.NQuad.createEdgePrototype
       2GB  4.05% 77.56%     5.48GB 11.09%  github.com/dgraph-io/dgraph/dgraph/cmd/bulk.(*mapper).addMapEntry
    1.63GB  3.29% 80.85%     1.63GB  3.29%  strings.makeCutsetFunc
```

(cherry picked from commit 3e52a8f)

dgraph/cmd/bulk/mapper.go

@@ -48,6 +48,7 @@ import (
 type mapper struct {
 	*state
 	shards []shardState // shard is based on predicate
+	mePool *sync.Pool
 }
 
 type shardState struct {
@@ -62,6 +63,11 @@ func newMapper(st *state) *mapper {
 	return &mapper{
 		state:  st,
 		shards: make([]shardState, st.opt.MapShards),
+		mePool: &sync.Pool{
+			New: func() interface{} {
+				return &pb.MapEntry{}
+			},
+		},
 	}
 }
 
@@ -125,6 +131,7 @@ func (m *mapper) writeMapEntriesToFile(entries []*pb.MapEntry, encodedSize uint6
 		x.Check(err)
 		_, err = w.Write(meBuf)
 		x.Check(err)
+		m.mePool.Put(me)
 	}
 }
 
@@ -182,9 +189,9 @@ func (m *mapper) run(inputFormat chunker.InputFormat) {
 func (m *mapper) addMapEntry(key []byte, p *pb.Posting, shard int) {
 	atomic.AddInt64(&m.prog.mapEdgeCount, 1)
 
-	me := &pb.MapEntry{
-		Key: key,
-	}
+	me := m.mePool.Get().(*pb.MapEntry)
+	me.Key = key
+
 	if p.PostingType != pb.Posting_REF || len(p.Facets) > 0 {
 		me.Posting = p
 	} else {