LevelDB backend: Recycle iterator in all keys fetch.

leveldb.go

@@ -169,7 +169,6 @@ func (ldbs *LevelDBSession) AllKeys(hash []byte) (keys chan []byte, n int, err e
 		ldbs.backend.mutex.RUnlock()
 		return
 	}
-	it.Close()
 
 	keys = make(chan []byte)
 
@@ -177,7 +176,6 @@ func (ldbs *LevelDBSession) AllKeys(hash []byte) (keys chan []byte, n int, err e
 		ldbs.backend.mutex.RUnlock()
 		defer ro.Close()
 		defer close(keys)
-		it := ldbs.backend.db.NewIterator(ro)
 		defer it.Close()
 		it.SeekToFirst()
 		for ; it.Valid(); it.Next() {