Discovery: don't wait joinThread when stopping

When a node stops, we cancel any ongoing join process. With #8327, we improved this logic and wait for it to complete before shutting down the node. However, the joining thread is part of a thread pool and will not stop until the thread pool is shutdown.

Another issue raised by the unneeded wait is that when we shutdown, we may ping ourselves - which results in an ugly warn level log. We now log all remote exception during pings at a debug level.

Closes #8359