Fixes BatchedStore interval calculation

There was a bug in refactoring and interval calculation in BatchedStore. This fixes it and simplifies the interval calculation. Also fixes the Scalding unit tests to use batch cover in input space instead of store space. Fixes #567
I guess I think this should not do the intersection at all here. You should basically be computing a `ExclusiveUpper[BatchID]` the snapshot at that point, and then the range of time from the moment after the last snapshot up to the upper bound that was requested.

Why is the intersection done here?

The meaning of this method is a bit unclear to me now. It looks like it can be trimmed even more. I can see why in the case of writing the batched output, we need to filter to be within batch boundaries, otherwise the partial batches will result in corrupted partial writes (potentially).

Let's talk in person a screen share on this one.