A new ClusterStateStatus to indicate cluster state life cycles

When the ClusterService applies a new cluster state, it is first assigned as the new active one and then all listeners are called. Some of ES's features sample the current state and try to take action on it (for example index a document). If that fails, they will wait for change in the cluster state and try again (for example, wait for a shard to start and try indexing again).

If you're unlucky you sample the state after it has been assigned as the "active" state but before all listeners has done the work. In this cases the action taken (i.e., indexing a doc) will still fail (as the shard is not yet started) but waiting for a new state may take a long time or timeout.

This commit adds a new ClusterStateStatus that allows to better track the stages a cluster state goes through (currently `RECEIVED`, `BEING_APPLIED` & `APPLIED`). This allows detecting that a cluster state is not yet fully applied and retry without waiting for a new state to arrive.

This commit also adds a utility class , ClusterStateObserver, to make this pattern slightly simpler and avoid common pit falls.