[STORE] Use DistributorDirectory only if there are more than one data direcotry

We don't need the overhead of DistributorDirectory if there is only a single directory
in the distributor.
Having a double assert here is weird, I think there should only be one `assert` and the `assertConsistency` method should either be renamed to `isConsistent` and return a boolean, or this assert should be removed, the `assertConsistency` method should return void and do the assert itself (since it has the `builder.toString()` message.