Buffer the publisher endpoint channel

This solves the problem described here: https://gist.github.com/tylertreat/72dd72a5b2ed0251443f
The problem is publisher messages are dropped because of artificial backpressure due to the
use of an unbuffered channel. Adding a small buffer alleviates the problem.
I'm debating on whether this should be higher. 5 seems very limiting on throughput. Even just bumping it up to 100 allows the publisher side to handle a tight publish loop without dropping messages and subscribers mostly keeping up.

Was there a reason you made this unbuffered @gdamore?