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
this does not look correct to me. There is a method that does this in Batcher.batchesCoveredBy does something related.

This code is not returning the timespan covered by the batcher in this minTime, maxTime range. It is extending the range of the bottom, but not the upper.

Can you add some comments as to what this method is supposed to do, because it looks to me like the semantics changed.