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.
At this point, the conversion is separate. No problem changing the order, but the state may, yes, be at a previous state relative to the log. But this is fine -- the one doesn't depend on the other in order. Changed anyway.