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.
it needs to set PrevExist.
I suggest it should be paired test with https://github.com/coreos/etcd/blob/master/etcdserver/server.go#L585
to see whether it could call the correct store method.