change fsm thread shutdown mechanism to make it more reliable
make catchup stoppable after each paxos value it has applied to the store
Multi_paxos.Full product_wanted should really mean full
always start a lease expiration thread when entering Slave_steady_state
keep lease expirations flowing in learner nodes
Multi_paxos.Full product_wanted should really mean full
Merge #155 remote-tracking branch 'refs/remotes/origin/cancel-fsm' into origin-1.7

Conflicts:
	src/node/catchup.ml