Guard against improper use-after-release of connection resources

[elprans]

The below seems to be a common (incorrect) pattern of use:

async with pool.acquire() as conn:
    # await conn.add_listener
    # await conn.prepare
    # etc

# Incorrect assumption that the listeners and prepared statements would continue to 
# work beyond the `conn` acquisition context

See #189 for an example issue.

To give a better hint that this is not a valid thing to do, we should check if conn has unclosed resources on it before releasing it. Possibly, behind a debug flag (either ASYNCPGDEBUG, or PYTHONASYNCIODEBUG, or both).

[elprans]

For prepared statements and cursors we want an InterfaceError to be raised, just like when an improper call to a connection method is made.