Use sync.Pool for posting lists in getInternal and getNew.

[martinmr]

Before:

ROUTINE ======================== github.com/dgraph-io/dgraph/posting.(*LocalCache).getInternal in /home/martinmr/go/src/github.com/dgraph-io/dgraph/posting/lists.go
    6.49GB    15.49GB (flat, cum) 11.68% of Total
         .          .    223:
         .          .    224:func (lc *LocalCache) getInternal(key []byte, readFromDisk bool) (*List, error) {
         .          .    225:   if lc == nil {
         .          .    226:           return getNew(key, pstore)
         .          .    227:   }
    1.13GB     1.13GB    228:   skey := string(key)
         .          .    229:   if pl := lc.getNoStore(skey); pl != nil {
         .          .    230:           return pl, nil
         .          .    231:   }
         .          .    232:
         .          .    233:   var pl *List
         .          .    234:   if readFromDisk {
         .          .    235:           var err error
         .     6.52GB    236:           pl, err = getNew(key, pstore)
         .          .    237:           if err != nil {
         .          .    238:                   return nil, err
         .          .    239:           }
         .          .    240:   } else {
         .          .    241:           pl = &List{
         .          .    242:                   key:         key,
    1.02GB     1.02GB    243:                   mutationMap: make(map[uint64]*pb.PostingList),
    4.34GB     4.34GB    244:                   plist:       new(pb.PostingList),
         .          .    245:           }
         .          .    246:   }
         .          .    247:
         .          .    248:   // If we just brought this posting list into memory and we already have a delta for it, let's
         .          .    249:   // apply it before returning the list.
         .          .    250:   lc.RLock()
         .          .    251:   if delta, ok := lc.deltas[skey]; ok && len(delta) > 0 {
         .          .    252:           pl.setMutation(lc.startTs, delta)
         .          .    253:   }
         .          .    254:   lc.RUnlock()
         .     2.48GB    255:   return lc.SetIfAbsent(skey, pl), nil
         .          .    256:}
         .          .    257:
         .          .    258:// Get retrieves the cached version of the list associated with the given key.
         .          .    259:func (lc *LocalCache) Get(key []byte) (*List, error) {
         .          .    260:   return lc.getInternal(key, true)

After:

ROUTINE ======================== github.com/dgraph-io/dgraph/posting.(*LocalCache).getInternal in /home/martinmr/go/src/github.com/dgraph-io/dgraph/posting/lists.go
    3.23GB     9.65GB (flat, cum) 11.14% of Total
         .          .    223:
         .          .    224:func (lc *LocalCache) getInternal(key []byte, readFromDisk bool) (*List, error) {
         .          .    225:   if lc == nil {
         .          .    226:           return getNew(key, pstore)
         .          .    227:   }
  851.02MB   851.02MB    228:   skey := string(key)
         .          .    229:   if pl := lc.getNoStore(skey); pl != nil {
         .          .    230:           return pl, nil
         .          .    231:   }
         .          .    232:
         .          .    233:   var pl *List
         .          .    234:   if readFromDisk {
         .          .    235:           var err error
         .     4.50GB    236:           pl, err = getNew(key, pstore)
         .          .    237:           if err != nil {
         .          .    238:                   return nil, err
         .          .    239:           }
         .          .    240:   } else {
         .          .    241:           pl = &List{
         .          .    242:                   key:         key,
  859.04MB   859.04MB    243:                   mutationMap: make(map[uint64]*pb.PostingList),
    1.56GB     1.60GB    244:                   plist: postingListPool.Get().(*pb.PostingList),
         .          .    245:           }
         .          .    246:   }
         .          .    247:
         .          .    248:   // If we just brought this posting list into memory and we already have a delta for it, let's
         .          .    249:   // apply it before returning the list.
         .          .    250:   lc.RLock()
         .          .    251:   if delta, ok := lc.deltas[skey]; ok && len(delta) > 0 {
         .          .    252:           pl.setMutation(lc.startTs, delta)
         .          .    253:   }
         .          .    254:   lc.RUnlock()
         .     1.88GB    255:   return lc.SetIfAbsent(skey, pl), nil
         .          .    256:}
         .          .    257:
         .          .    258:// Get retrieves the cached version of the list associated with the given key.
         .          .    259:func (lc *LocalCache) Get(key []byte) (*List, error) {
         .          .    260:   return lc.getInternal(key, true)

---

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[martinmr]

This PR depends on #3767