publication "locking"

We need a way for a publisher to "lock" a topic so that only they can publish to it. This is needed to prevent rogue processes, or multiple instances of the same application, from publishing to the same channel. The basic idea would be something whereby the publisher application acquires a lock for the channel such that only it can publish to the channel while it holds the lock.

@patrick.godwin suggests that maybe kafka transactions provide an interface like this.