AsyncStore shutdown can leak threads

AsyncStore stop() should take care that all threads (coordinator and workers) are shut down. Right now there could be worker threads left hanging and the coordinator thread waiting forever for them to end.

coordinator.join(shutdownTimeout) in AsyncStore.stop() could time out and both coordinator and worker thread will leak in this case.

We must ensure the coordinator is terminated and the ExecutorService used for worker threads is also shut down when we exit AsyncStore.stop().