Make greptimedb as a kv backend for greptimedb

[CookiePieWw]

A interesting idea, does not matter seriously.

Currently greptimedb support pg protocol, and can serve as a metakv backend for greptimedb itself (haven't tested actually, maybe need some compatible adjustions). #5208 will introduce leader election for multiple metasrvs, which requires pg_advisory_lock. So at least we need a imitation of advisory lock to make greptimedb as a kv backend for greptimedb.