*For comment only* 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.

Outstanding is to write some tests around ContentBuffer. This is for initial comments ahead of that.