Wait until all queues are empty. This method waits until all non‐paused queues are empty without disabling the queues (and may thus wait indefinitely). The queues and/or the thread pool may be either enabled or disabled when this method is called. This method may be called on a stopped or started thread pool. Note that drain does not attempt to delete queues directly. However, as a side‐effect of emptying all queues, any queue for which deleteQueue was called previously will be deleted before drain returns. Note also that this method waits by repeatedly yielding.