straighten catchup interface
correct i and vo to start election with when starting a node

Conflicts:
	src/node/catchup.ml
	src/node/node_main.ml
	src/paxos/slave.ml
backport from 9a4d8207b130880b4be3ad76afc5ddc7d9df1c07
ensure the previous value being pushed really is the previous
backport from 6549725b6f1c3b108c9dda4de7c8ae211e360e43
Merge pull request #226 from Incubaid/1.6-backport

1.6 backport