ConnectionProxy doesn't detach conn when leaving async context

[touilleMan]

Originally reported by @vxgmichel

connection pool's acquire context manager returns a connection proxy object that can still be used after we have left the context manager (instead of raising an error)

This is because the triopg version of the connection proxy is missing an additional cleanup routine compared to what is done in asyncpg:
https://github.com/MagicStack/asyncpg/blob/7cb4e70d88d165273997d914280c6d109fbbc8f6/asyncpg/pool.py#L78-L84