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
[STATE] Observe cluster state on health request

Today we use busy waiting and sampling when we execute HealthReqeusts
on the master. This is tricky sicne we might sample a not yet fully applied
cluster state and make a decsions base on the partial cluster state. This can
lead to ugly problems since requests might be routed to nodes where shards are
already marked as relocated but on the actual cluster state they are still started.
Yet, this window is very small usually it can lead to ugly test failures.

This commit moves the health request over to a listener pattern that gets the actual
applied cluster state.

Closes #8350