Add stream multiplexing

[rekmarks]

Sometimes, we need to transmit different kinds of traffic over the same stream. Currently, we manage this by layering a "message envelope" abstraction on top of our streams, where different envelopes correspond to different kinds of traffic. In order to encapsulate this complexity within the stream implementations themselves, we should implement multiplexing for our streams.

[rekmarks]

See #218 for PR stack. #223 will close this issue, #224 is a mostly orthogonal follow-up.