Ceph

Right now the monitor calls tick() on all the PaxosService implementations when it's doing slurping. This introduces a race, in the case that there is slurping going on which has changed the on-disk data without yet putting it back into a consistent state.

There is a quick fix in wip-tick-slurping-race, but we should come up with a stronger solution (integrate slurping into the Paxos state system, probably?) for master. (Or, hopefully, merge Joao's single-paxos stuff soon enough that we don't need to worry about it.)