plumbing: Turn log-firehose into a pubsub server.

There are at least two sinks for log messages:

1. The dashboard.
2. Longer-term storage of ArchiveBot log messages.

(1) exists; (2) does not.  I don't really want to build (2) myself, so I
need to provide some way for people to get the messages.  ZeroMQ pubsub
seems to do pretty well.