JoinableQueue是由Queue派生出来的一个类，与Queue不同，JoinableQueue有task_done()和Join()方法。
官方文档：
task_done()
Indicate that a formerly enqueued task is complete. Used by queue consumer threads. For each get() used to fetch a task, a subsequent call to task_done()tells the queue that the processing on the task is complete.
If a join() is currently blocking, it will resume when all items have been processed (meaning that a task_done() call was received for every item that had been put() into the queue).
Raises a ValueError if called more times than there were items placed in the queue.
join()
Block until all items in the queue have been gotten and processed.
The count of unfinished tasks goes up whenever an item is added to the queue. The count goes down whenever a consumer thread calls task_done() to indicate that the item was retrieved and all work on it is complete. When the count of unfinished tasks drops to zero, join() unblocks.
task_done()表示队列中的某个任务已经完成了
join()会一直阻塞直到队列中的所有任务已经被获取并处理。一旦任务被加入队列，任务计数会加1，一旦task_done()任务计数会减1，如果任务计数减为0，则join()解除阻塞
