Adding a capability to manipulate MyRocks data dictionary

[yoshinorim]

Sometimes it is useful if we can manipulate data dictionary. One typical use case is fixing rocksdb_ddl and frm file inconsistency. The inconsistency may happen when a temporary table existed in MyRocks before finishing myrocks_hotbackup's WAL copy but was dropped before copying frm file. Then the temporary table info is stored in rocksdb_ddl but frm file does not exist in destination. To fix such inconsistency, currently it is necessary to copy frm file to data dir then to run drop table.