asyncio, Tulip issue 220: Merge JoinableQueue with Queue.

Merge JoinableQueue with Queue. To more closely match the standard Queue,
asyncio.Queue has "join" and "task_done". JoinableQueue is deleted.

Docstring for Queue.join shouldn't mention threads.

Restore JoinableQueue as a deprecated alias for Queue. To more closely match
the standard Queue, asyncio.Queue has "join" and "task_done".  JoinableQueue
remains as a deprecated alias for Queue to avoid needlessly breaking too much
code that depended on it.

Patch written by A. Jesse Jiryu Davis <jesse@mongodb.com>.