Prevent unnecessary safecopy in iterator parseKV (dgraph-io#971)

The previous implementation of parseKV would copy over the value slice
every time we iterate, which is not necessary. The value slice should be
copied only when we return the result of point lookups. The proposed patch
removes safeCopy call from the iterator.

table/iterator.go

@@ -180,8 +180,8 @@ func (itr *blockIterator) parseKV(h header) {
 			"Pos:%d Len:%d EndOffset:%d Header:%v", itr.pos, len(itr.data), valEndOffset, h)
 		return
 	}
-	// TODO (ibrahim): Can we avoid this copy?
-	itr.val = y.SafeCopy(itr.val, itr.data[itr.pos:valEndOffset])
+
+	itr.val = itr.data[itr.pos:valEndOffset]
 	// Set pos to the end of current entry.
 	itr.pos = valEndOffset
 }

txn.go

@@ -420,7 +420,7 @@ func (txn *Txn) Get(key []byte) (item *Item, rerr error) {
 	item.meta = vs.Meta
 	item.userMeta = vs.UserMeta
 	item.db = txn.db
-	item.vptr = vs.Value // TODO: Do we need to copy this over?
+	item.vptr = y.SafeCopy(item.vptr, vs.Value)
 	item.txn = txn
 	item.expiresAt = vs.ExpiresAt
 	return item, nil