Add support for sqlite3_unlock_notify

[typeless]

It's likely required for fixing go-gitea/gitea#2040.

I have an experimental implementation at #439. It has not much polished yet but could be useful as a reference.

[gjrtimmer]

Reference #439

[kjellkvinge]

#439 is merged. Wouldn't that merge close this?

[typeless]

@kjellkvinge yes, this should be closed.

[typeless]

@kjellkvinge yes, this should be closed.