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