straighten catchup interface
correct i and vo to start election with when starting a node
ensure the previous value being pushed really is the previous
ensure a MasterSet is pushed after reaching consensus on another value when running for master
Merge remote-tracking branch 'refs/remotes/origin/start-i-after-catchup' into 1.7

Conflicts:
	src/node/catchup.ml
	src/paxos/slave.ml