`ClusterService` is responsible of updating the cluster state on every node (as a response to an API call on the master and when non-masters receive a new state from the master). When a new cluster state is processed, it is made visible via the `ClusterService#state` method and is sent to series of listeners. Those listeners come in two flavours - one is to change the state of the node in response to the new cluster state (call these cluster state appliers), the other is to start a secondary process. Examples for the later include an indexing operation waiting for a shard to be started or a master node action waiting for a master to be elected.
The fact that we expose the state before applying it means that samplers of the cluster state had to worry about two things - working based on a stale CS and working based on a future, i.e., "being applied" CS. The `ClusterStateStatus` was used to allow distinguishing between the two. Working with a stale cluster state is not avoidable. How this PR changes things to make sure consumers don't need to worry about future CS, removing the need for the status and simplifying the waiting logic.
This change does come with a price as "cluster state appliers" can't sample the cluster state from `ClusterService` whenever they want as the cluster state isn't exposed yet. However, recent clean ups made this is situation easier and this PR takes the last steps to remove such sampling. This also helps clarify the "information flow" and helps component separation (and thus potential unit testing). It also adds an assertion that will trigger if the cluster state is sampled by such listeners.
Note that there are still many "appliers" that could be made a simpler, unrestricted "listener" but this can be done in smaller bits in the future. The commit also makes it clear what the `appliers` and what the `listeners` are by using dedicated interfaces.
Also, since I had to change the listener types I went ahead and changed the data structure for temporary/timeout listeners (used for the observer) so addition and removal won't be an O(n) operation.
b6cbcc49ba