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?
I think the directories are sorted using lexicographic order, but we should use numeric sorting according to the comment above. Also, the sort order isn't specified in the documentation. I'm suggesting to explicitly sort the entries.