Multi backend proposal

Hi Ask,

This is initial version of the backend allows to publish tasks to multiple workers and receive multiple replies. Of course, it's not ideal, and contains some hacks, but I'll improve it; and I'm asking for the guidance from your side since you're actually the person who knows what should be in Celery, and what shouldn't.

In the next comment I'll show some examples how it works.

(I made a checkout from v3.1.11 tag, probably that's why all those commits are here, ignore everything except the latest please)