[dev] Tracking issue for database persistence

[skyzh]

Currently, we simply delete table files after we drop the last reference Arc to TableInner. This causes several issues.

• We had to forget (leak) table manually, thus causing leak sanitizer complain. https://github.com/tikv/agatedb/pull/60/checks?check_run_id=1452618735
• By using mem::forget, we also have to manually close underlying file objects.

[skyzh]

The leak issue has been fixed in 0ce60db

Current solution is that:

• We add a save_after_close atomic bool in related structure.
• If we need this file in next database load, we mark this structure as not to delete.
• If save_after_close is set when dropping Wal or SST, we don't delete the corresponding file.