CreateTSComparator should behavior identical with rocksdb builtin timestamp comparator.

NewVersionDBOpts returns the options used for the versiondb column family.

OpenVersionDB opens versiondb, the default column family is used for metadata, actually key-value pairs are stored on another column family named with "versiondb", which has user-defined timestamp enabled.

OpenVersionDBAndTrimHistory opens versiondb similar to `OpenVersionDB`, but it also trim the versions newer than target one, can be used for rollback.