Mechanism to provoke an unscheduled heartbeat.
On tor-talk, Lars Noodén suggests that we should have some way to get a heartbeat message when none is scheduled. Seems like a fine idea to me! All of the good signals seem to be taken though. (HUP, USR1, USR2). We could add it to HUP, but that wouldn't let you get just the heartbeat, given all the other stuff that HUP does.
We could add a controller-level signal, but that's not quite so convenient.
Once we're done bikeshedding the interface, the implementation should be simple.