Problem: consistency is not checked on startup

if the memiavl directory is dirty, we might get errors later down the stream.

do some checks on startup to find issues early on:
- tmp snapshot directories
- snapshots that's later than the "current" one
- the "current" snapshot do exists.
- wal covers all the snapshots.

Start by tracing the memiavl startup path and how it opens the memiavl directory, snapshots, and WAL. Check how temporary snapshots, the current snapshot, later snapshots, and WAL coverage are represented. Done means startup detects and reports each listed inconsistency before later processing can fail.

Written by the indexing model from the issue text.