WIP: migration from pre-v0.5

This is still quite rough, but it would be nice to get some feedback on the general direction and resolve some of the questions noted in TODOs.

Known issues:
- Migrating with snapshots does not work on master due to https://github.com/coreos/etcd/issues/1266
- Multi-node cluster migrations end up shifting the store indexes forward by N where N is the size of the cluster