Discovery: remove any local state and use clusterService.state instead

At the moment, ZenDiscovery contains a local copy of the disco nodes plus a flag that indicates whether the local node is master or not. This is redundant as the same information is stored in the cluster state. Have duplicate copy can lead to unneeded concurrency issues. This PR removes the duplication.

The PR introduces a tighter control of the background joining thread to make sure it is started and stopped together with any cluster state changes. This solves potentially concurrency bugs where a joining thread may fail to start.

Last we add a couple of safety checks to make sure that if a nodes receives a cluster state from a new master while actively trying to join another one (or electing itself) we go back to pinging to actively join it.