migrate: Add a migration tool to go from etcd v0.4 -> v0.5 data directories

Still a WIP, because actually restarting a cluster off of these converted directories doesn't work yet -- but individually the nodes agree on the data. Looking for comments.
@barakmich when append entry, it needs to send `prevLogIndex` and `prevLogTerm` to let follower check that the append is valid and has no conflict. So we need to put a entry before ents5 with correct index and term.