The race condition occurs when the restart is faster than
the cleaning up. With this commit the restart will perform
the cleanup if it was not done beforehand.

In addition to cleaning when ranch:stop_listener/1 is called, we
also need to clean when we detect the supervisor is going away
for normal reasons, because the supervisor might be in another
application's supervision tree.

Note that there might be a short delay in this case before the
cleanup is done, due to using monitors for detection.

This allows graceful draining of connections, updating transport
options on a running listener without having to drop connections
and other similar scenarios.

Note that when updating transport options the listener must be
suspended which means that new connections will be rejected until
the listener is resumed.

In some cases the ranch_server:get_connections_sup/1 call could
still succeed depending on timing.