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.
ack. but i still want to change the order to make it the same as the real one that may generate.
we may add sanity check in wal, and i want it to be 100% safe through following certain order.