Improve code to update related revision tables

* Changed method splitUpdate; it will now accept a column prefix & return only
  columns matching that prefix (updateRelated can fetch it's own using
  splitUpdate with the appropriate prefix)
* Changed updateRelated: it'll now accept (all, not just related) $changes,
  (whereupon it can call splitUpdate itself) & old $row
* PostRevisionStorage::updateRelated is no longer unimplemented: it will be run
  if the changed columns are in $allowedUpdateColumns (similar to update())
* Update will now check for allowed columns in static (we're on PHP5.3 already)
  instead of self, so specific implementations can overwrite/add their own.

Change-Id: I31eacc2839f03d0200cce2f41bb9a17745537f37
Merge "Improve code to update related revision tables"