rocksdb.cons_snapshot_read_committed testcase failed sometime

rocksdb.cons_snapshot_read_committed testcase failed sometime, but can retry-pass.

```

[ 60%] rocksdb.commit_in_the_middle_ddl 'write_prepared' w4 [ pass ] 214476 [ 60%] rocksdb.cons_snapshot_read_committed 'write_prepared' w4 [ fail ] Test ended at 2019-07-24 01:02:23 CURRENT_TEST: rocksdb.cons_snapshot_read_committed — /builds/gdb-cluster/myrocks/artifacts/mysql/mysql-test/suite/rocksdb/r/cons_snapshot_read_committed.result 2019-07-10 14:26:44.000000000 +0300 +++ /builds/gdb-cluster/myrocks/artifacts/mysql/mysql-test/var/4/log/cons_snapshot_read_committed.reject 2019-07-24 04:02:23.140611298 +0300 @@ -10,13 +10,13 @@ connection con2; select * from information_schema.rocksdb_dbstats where stat_type='DB_NUM_SNAPSHOTS'; STAT_TYPE VALUE -DB_NUM_SNAPSHOTS 0 +DB_NUM_SNAPSHOTS 1 connection con1; COMMIT; connection con2; select * from information_schema.rocksdb_dbstats where stat_type='DB_NUM_SNAPSHOTS'; STAT_TYPE VALUE -DB_NUM_SNAPSHOTS 0 +DB_NUM_SNAPSHOTS 1

```