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.
5 was kind of an arbitrary choice, but it matches what's used in bus and star and it fixes the problem I was having with dropped messages.