Some places forget to call `assert` to check the return value of `AsynchronousListenerBus.waitUntilEmpty`. Instead of adding `assert` in these places, I think it's better to make `AsynchronousListenerBus.waitUntilEmpty` throw `TimeoutException`.

Author: zsxwing <zsxwing@gmail.com>

Closes #6550 from zsxwing/SPARK-8001 and squashes the following commits:

607674a [zsxwing] Make AsynchronousListenerBus.waitUntilEmpty throw TimeoutException if timeout