Allow asynchronous publishers/consumers

[cstorey]

Currently, we require that an event is completely acked before returning to the publisher, or require that a connection go into consumer mode entirely. This limits both throughput and flexibility.

[cstorey]

Incidentally, because there's currently no way for us to know how big a message is when reading it, we'll need some sort of trivial framing mechanism. Something like that used in uswitch's baldr would do.