Use saturating arithmetic in UpdateForBatchPosterSpending #740

UpdateForBatchPosterSpending includes some arithmetic that should be safe from overflow, given the realistic bounds on timestamps. But a more careful approach, which we should adopt, is to make the code unconditionally safe against weird timestamp values, by adopting saturating arithmetic or adding checks.

This is technically speaking a change to consensus code, but there should be no difference in any practical case that has been encountered.