roll back default where zmq high water mark = 0

This is a follow-up to INDY-366. In that ticket, we changed the default FIFO queue in ZMQ to be unlimited. This makes certain problems less likely, but it also masks problems for a lot longer. I would be more comfortable that we were truly robust if:

1. The high water mark were finite (e.g., 100,000).
2. We had a way to detect that we are nearing the high water mark, and that we would report a graceful error that makes this condition easy to diagnose.
3. We implemented logic in STP to discard old messages rather than new ones. (Desirable but not necessarily required.)
4. If we exceed the high water mark, we exit the daemon process gracefully rather than crashing due to an out-of-memory condition.

This ticket is to track the work in items 1-4. Possibly 3 could be omitted.