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

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

A few semantic changes result from this:
* If multiple pages are edited 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 the same page is edited twice in a request, the WikiPage hook
  argument will reflect the last request edit, not always the edit
  that fired the hook.

Bug: T120718
Change-Id: I9429f29e5a90f24e4d7af5797a80e63a9cc34146