RFC 697: Run schema+data migrations over multiple versions in the `migrator`

See [RFC 697: Multiple version upgrades](https://docs.google.com/document/d/1eNiwzWZmfpvzFhs9IDcM7Sbchv_Hm8Uj-6bI-3TDlvg).

Add a subcommand to the migrator service that interleaves the application of schema and out-of-band migrations. An initial implementation of this behavior can be as simple as `migrator upgrade {target_version}`.