Avoid noisy RejectedExecutionExceptions during StandaloneSchedulerBackend shutdown

While it is in the process of shutting down, the StandaloneSchedulerBackend might throw RejectedExecutionExceptions when RPC handler `onDisconnected` methods attempt to submit new tasks to a stopped executorDelayRemoveThread executor service. We can reduce log and uncaught exception noise by catching and ignoring these exceptions if they occur during shudown.