fix(Dgraph): Don't store start_ts in postings. (#6206) (#6212)

Clear out the start_ts field in the postings of deltas before they are
marshalled to avoid storing that field in disk. This field is only meant
to be used during in-memory processing.

Related to https://discuss.dgraph.io/t/start-ts-not-being-cleared-before-postings-are-written-to-disk/9146

(cherry picked from commit fbbd731)

posting/list.go

@@ -513,9 +513,12 @@ func (l *List) addMutationInternal(ctx context.Context, txn *Txn, t *pb.Directed
 
 // getMutation returns a marshaled version of posting list mutation stored internally.
 func (l *List) getMutation(startTs uint64) []byte {
-	l.RLock()
-	defer l.RUnlock()
+	l.Lock()
+	defer l.Unlock()
 	if pl, ok := l.mutationMap[startTs]; ok {
+		for _, p := range pl.GetPostings() {
+			p.StartTs = 0
+		}
 		data, err := pl.Marshal()
 		x.Check(err)
 		return data
@@ -586,6 +589,7 @@ func (l *List) pickPostings(readTs uint64) (uint64, []*pb.Posting) {
 					deleteBelowTs = effectiveTs
 					continue
 				}
+				mpost.StartTs = startTs
 				posts = append(posts, mpost)
 			}
 		}