Support identifying encoding of relayed zmq stream

Depending on the scope of what kind of relayed streams we're going to relay through RouterServer, this may be a very hard problem to solve.

If we *just* support relaying zmq streams from other RouterServer instances, we can ask the upstream RouterServer for the encoding and pass it down. If we support arbitrary zmq streams, we'd have to know the encoding in advance and likely pass it in when we create the zmq stream.