RabbitMQ support

[geofft]

For certain applications it would be nice to support ​RabbitMQ, a message queuing system. The point of a message queuing system is to do things like request a task be done and return, and have an asynchronous task handler process the task, and not have _two_ handlers pick up the same task, even if they're on different machines.

Much as with memcached support, the big issue here is ACLing. RabbitMQ supports some amount of "vhost" support with ACLs, so maybe that will work. And again, the other two big issues are sane multi-server support that works with our environment without introducing a SPOF, and limiting resource consumption (although I expect this to be less of an issue with RabbitMQ).