Interrupting session expiration task on shutdown can leave orphaned entries in cache

Shutting down the server (or undeploying an application) currently performs an ExecutorService.shutdownNow(), which interrupts the session expiration task. If this task in the process of removing entries for an expired session, and the backing cache in non-transactional, this can result in stale cache entries which are no longer accessible.

Instead we should use ExecutorService.shutdown(), and make the expiration task sensitive to shutdown state.