Validate and specify PoolConnection's behavior in the face of unexpected drops and general panics

[abonander]

Context: #110 (comment)

PoolConnection should probably be sure to clear the inner connection's buffer before releasing it back to the pool, and maybe on panic just kill the connection entirely (because e.g. it could be in an unclosed transaction).

Currently it does... none of that, which is actually a bit concerning (I wrote it so I take full responsibility):

sqlx/sqlx-core/src/pool/conn.rs

Lines 72 to 81 in 80f402b

	impl<C> Drop for PoolConnection<C>
	where
	C: Connection + Connect<Connection = C>,
	{
	fn drop(&mut self) {
	if let Some(live) = self.live.take() {
	self.pool.release(live.float(&self.pool));
	}
	}
	}