Drop task management from trio.Queue #321

Specifically this means deprecating and removing task_done and join.

I've been tempted to do this for a while b/c they're complicated, very rarely useful (as in: I'm not sure they have any use at all in Trio), and deeply inelegant (why is there an independent synchronization mechanism bolted onto Queue?). The only reason they're there in the first place is to match stdlib queue.Queue. Final straw: https://bugs.python.org/issue14976#msg301173