Clean up page seeking a bit.
* 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.