write 'current datastore format' to repo

If we want to make it easy for ipfs to move between and change its repo storage formats, we should store our current configuration somewhere on disk in the repo. That way if the user changes the config, we can see old vs new and move data accordingly.

We can also in the shorter term just detect that things arent the same and ask the user to run some external migration tool in that case.

The impetus for this comes from me wanting to be able to change the sharding of the flatfs directories more easily.