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
@johnynek, this code refactors what was in master, it is based on what you had in the original PR #551. The logic is the same as what is in master, AFAICT, except this refactors readBatched to be used to now produce the delta stream for leftJoin. I moved the Scalding.limitTimes to be after mergeBatched call since the limitTimes there was taking the available time span that was in store space and thus filtered out all of the data, since the data is in delta/input space.