Round-robin through a list of keys

[GlenRSmith]

I'd like to see the (routing) key property be allowed to be a list, and have the messages sent round-robin with the values in the list.

I propose this to provide a fairly simple solution to a fairly common problem: a RabbitMQ queue represents a single Erlang process, which is a single thread, only serviced by a single processor core.

With this solution in place, the logstash instance with a RabbitMQ output could have a list of routing keys from e.g. "logstash-1" through "logstash-N" feeding an exchange deployed on a box with N cores, and allow full use of the processor power of the server.

[smetj]

@GlenRSmith The idea is good but you should use x-consistent-hash and x-modulus-hash exchanges instead which is already referenced in #29