refactor: don't keep the notifiers backlog handler in the connection state

refactor: don't keep the notifiers backlog handler in the connection state

Just keep the queue in the state and special-case its handling in the
`_notify_handler` connection method instead of registering a standard handler.
Set the queue to None to signify that we are in the `notifies()` generator.

This way we don't need the awkward weak-self + class method to avoid a
reference loop and to dereference the connection weak reference another
time, as we just did in `_notify_handler()`. Setting the queue to None
also feels cleaner than adding/removing the handler.

Relates to #975.