Fabric8 native leader election

[wind57]

No description provided.

[wind57]

The idea here goes like this:

• there is a CachedSingleThreadScheduler podReadySelfShutDownScheduler that will shut down itself when there are no tasks for it to run
• we issue the call podReadySelfShutDownScheduler::podReady that returns a CompletableFuture
• the podReady method schedules the podReadySupplier to run and returns a ScheduledFuture
• When ScheduledFuture is either done or fails, we complete the CompletableFuture with that status. For example:

if (podReadySupplier.getAsBoolean()) {
    podReadyCompletableFuture.complete(null);
}

• the attachShutDownHook makes sure that if the resulting CompletableFuture is completed, we also stop the ScheduledFuture, thus shutting down the executor; thus not leaving anything hanging.