Converger interface

I briefly considered making this method guarantee something about a single convergence cycle, but then I though about what would happen in a hypothetical future situation where some machines blow up during a load spike:

- Nova tells us we're down two machines, in two separate events
- Our metrics tell us something bad is going on
- Third party thing tells us something bad is going on

All of these get translated into "schedule convergence" messages on a queue; but we really do only want one actual convergence cycle (which may, in turn, schedule other convergence cycles).