Hardening of cluster member leaving path, see #3309

* ClusterSingletonManagerSpec revealed a bug in Gossip merge, which
  caused a removed member to come back into the member set after
  a gossip merge.
* Removed leader commands for Shutdown and Exit
* Member shutdown itself  when it sees itself as Exiting
* Singleton cluster with status Exiting will shutdown itself,
  in case the Exiting gossip never arrives
* Exiting member not part convergence check
* Exiting member is removed by leader (on convergence) when the
  exiting member is in the unreachable set, i.e. sucessfully shutdown
* Reverted the change made for #3266, i.e. Exiting is
  detected as unreachable again.
* Removed entry should have higher prio than Exiting and Down in
  Gossip merge