* Guarantee pcm_start and pcm_end stay in range (not just move in
   the right direction).
* When we fail to find a page in the interval, back up by increasing
   chunk sizes just like op_get_prev_page_serial().
* Eliminate the special case for the last page in the interval.
* Force a straight bisection if we're backing up, but not decreasing
   the interval size rapidly enough, to limit the worst-case.
  This is guaranteed not to affect the first two iterations, so it
   has minimal impact on seeking in the normal case.