Convert page creation to using startAtomic()/endAtomic()

Convert page creation to using startAtomic()/endAtomic()

A few semantic changes result from this:

* If multiple pages are created in a request, the updates happen
  in the same order relative to each other, but all in one second
  step instead of after each page edit.
* If an extension set some extra Status info or errors via the
  PageContentInsertComplete hook, they will not be seen by the
  caller (unless it was a CLI script possibly). Few callers use
  $status at all, and I did not see any that mutated it. Since
  the page is already committed when this hooks run (as has always
  been) they cannot veto edits and callers do not care or know what
  to do with random hook-set status errors; there was never much use
  in changing the Status anyway.

Bug: T120718
Change-Id: Ieba35056be31b2f648c57f59d19d3cbbe58f1b05