Check workers before broadcasting heartbeats.

Add a new option to check our workers before we broadcast a heartbeat.  If they are all busy, we broadcast a flatline so any clients listening to us will know not to send us any more work.  If we happen to recover, the server will start broadcast heartbeats when a worker becomes available.

The default behavior is that this setting is disabled, so default behavior does not change.

Also, the TTOU signal will inform you of how many workers are busy.