Add migration tool for 0.7 -> 0.8

- Removed all the deprecated stuff from datastore/shard
- Add data_migrator and test
- Update server to create and run the migrator
- Add the old level_db_shard.go to read old data
Move migration code to its own package.

* Moved over the data migrator
* Moved over the old leveldbshard code and updated datastore/shard so old code could reference everything.
Move migration from starting automatically to an API endpoint.
Update the migrator to mark shards as migrated

Will write a file into the shard directory. If a migration is called again, it will skip any shard that has a migration marker in the directory.