Figure out datastore plugin support

[magik6k]

While this isn't really an issue now, but it soon may be as datastore plugin support got merged recently.

With how the go plugin system works, we won't be able to load the plugins here, so we need to figure out a way of working with custom datastores:

• Rewrite migrations as a go-ipfs plugin
  • We'd need to provide builds of fs-repo-migrations for every version of go-ipfs, so the downgrade path still works.
• Make migrations easy to rebuild with new datastore support
  • This should be fairly doable with extracting repo/fsrepo and parts of the go-ipfs plugin subsystem