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  change came up to repro of #567 and internally as well. This is used in the tests. Why do we shift the input to the next batch for the tests? It does not seem to correspond to what we do in actual summingbird-scalding jobs based on debugging. We use this method to request a computation for a particular time interval [here](https://github.com/twitter/summingbird/blob/1b630690a8efe554ab3fabc9b60965c2c95e2e83/summingbird-scalding-test/src/test/scala/com/twitter/summingbird/scalding/ScaldingLaws.scala#L84) which seems to request times in store space, but in our scalding jobs we request the input in delta/input space.