746 migration

note quite done. Still have to add the flagging that a shard has been migrated so it won't be done again.

I'm also considering making data migrator only run if they start up influx with an argument to do it. That way, people could upgrade, start things up and then create shard spaces before migrating data. Thoughts?
In this case, lexigraphical and numerical are the same since the directory names are zero padded. However, I didn't know that the sort order wasn't guaranteed in ReadDir, I just assumed based on what I was getting back.

At the end of the day, it's not super important which get migrated first. The shard id order isn't even a guarantee on which shards have newer data (for instance if they backfill after writing a bunch of current stuff). Ok if I leave as is for now?