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
can we change the name to this method:
`readAfterLastBatch` or something? It is reading from input all the data AFTER the most recent batch on disk, right?