perf(core): Introduce mutable layer struct in list

posting/index.go

@@ -725,15 +725,10 @@ func (r *rebuilder) RunWithoutTemp(ctx context.Context) error {
 						return err
 					}
 					pl.CommitTs = item.Version()
-					for _, mpost := range pl.Postings {
-						// commitTs, startTs are meant to be only in memory, not
-						// stored on disk.
-						mpost.CommitTs = item.Version()
-					}
 					if l.mutationMap == nil {
-						l.mutationMap = make(map[uint64]*pb.PostingList)
+						l.mutationMap = newMutableLayer()
 					}
-					l.mutationMap[pl.CommitTs] = pl
+					l.mutationMap.insertOldPosting(pl)
 					return nil
 				})
 				if err != nil {