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.
if ExpireTime is Permanent, it should be translated to 0 here.
reference:
https://github.com/coreos/etcd/blob/master/store/node.go#L36
https://github.com/coreos/etcd/blob/master/etcdserver/server.go#L567