storage: use Update for changeset processing

Batch is only relevant in multithreaded context, internally it'll do some
magic and use the same locking/updating Update does, so it makes little sense
for us. This doesn't change benchmarks in any noticeable way.

pkg/core/storage/boltdb_store.go

@@ -92,7 +92,7 @@ func (s *BoltDBStore) PutBatch(batch Batch) error {
 func (s *BoltDBStore) PutChangeSet(puts map[string][]byte) error {
 	var err error
 
-	return s.db.Batch(func(tx *bbolt.Tx) error {
+	return s.db.Update(func(tx *bbolt.Tx) error {
 		b := tx.Bucket(Bucket)
 		for k, v := range puts {
 			if v != nil {