Batch PA writes into one per channel day

Many channels have lots of repeats of the same programme throughout
the day (think weather every 30 minutes). This can lead to some very
write-heavy periods with large items being updated repeatedly. Here
we are batching the writes such that an item and its hierarchy is
written at most once per channel day.
PA brand and series summaries must be written after content
Switch PaEmptyScheduleProcessor to use ContentHierarchyAndSummaries
Static import of Preconditions
Merge pull request #416 from atlasapi/batch-pa-channel-day

Batch PA writes into one per channel day