[WIP] Fix SQLite 'database table is locked' problem

[typeless]

Fixes #2040

CAVEAT: the PR hasn't fixed the issue yet.

According to the documentation, the upper layer has to handle the SQLITE_LCOKED status code reported by the underlying sqlite_unlock_notify API as well. The proper approach to fix it in upper layers still needs investigation but the basic mechanism described in the following link has been implemented in the PR.

If this can eventually fix the issue, I will propose the PR to upstream.

Reference: https://sqlite.org/unlock_notify.html.

[andreynering]

You should consider sending this PR upstream to https://github.com/mattn/go-sqlite3 instead for two reasons:

• No one here have enough knowledge of SQLite internals to review this. @mattn, on the other side, will be able to tell if what you are trying to do is right.
• We usually should not change vendor/ folder directly. (We would have to manually rebase future upstream changes).

[typeless]

@andreynering Understood and agreed. I will continue working on it later.

[andreynering]

Closing this for now. #2040 exists to track this bug