PumpkinDB: Problem: wall clock going back in time between restarts will break HLC guarantee

Basically, it means that after a restart values returned by HLC will be less than those generated before.

Proposed solution: let timestamp module persist last generated HLC.

Letting it store in the same database is rather intricate: there might be no write txn, the write txn might never succeed, etc. So I propose that PumpkinDB maintains a separate “meta” database within the environment with its own transactions.

Any better solution (that would involve I/O penalty?)