Track all connection is dropped in deadpool-sqlite

[xuxiaocheng0201]

Fixes #330
Fixes #468

[bikeshedder]

Awesome! ❤️

Even though I'd prefer if it was actually a feature of deadpool itself. I'll have to tinker with this a bit. I'll look into that very soon. 👍

[bikeshedder]

I did not forget about this PR. I just want to push this to the deadpool crate rather than making this a deadpool-sqlite exclusive feature.

[xuxiaocheng0201]

We cannot directly track whether M::Type in Object<M> has indeed been dropped, as it is usually an external type and may not necessarily implement Drop. My solution is to track deadpool_sync::SyncWrapper, because only it is controllable, so this may not generalize to the entire deadpool, but only to all managers who use SyncWrapper.