deadpool-sqlite ConnectionWrapper is not async-safe

[SergioBenitez]

ConnectionWrapper wraps a rustqlite::Connection which implements a Drop that may perform synchronous I/O:

impl Drop for Connection {
    #[inline]
    fn drop(&mut self) {
        self.flush_prepared_statement_cache();
    }
}

Thus, when (the last of the clones of) ConnectionWrapper is dropped, synchronous I/O will be performed in what is surely to be an async context. This is, of course, logically incorrect behavior that can lead to quite a few problems including panics.

In general, wrapping a synchronous connection correctly (or any other I/O object) is incredibly error-prone. Please see our implementation in rocket_sync_db_pools where we've painstakingly tried to address all of the relevant issues: https://github.com/SergioBenitez/Rocket/blob/693f4f9ee50057fc735e6e7037e6dee5b485ba10/contrib/sync_db_pools/lib/src/connection.rs.

[bikeshedder]

Good catch. I didn't think of this.

I did hesitate to add any sync database drivers to deadpool for that exact reason. I should've known better than releasing a half-baked implementation of this. 😞

Please review #115 which fixes this problem. Thanks a lot!