fix and extend row ordering rules

We now schedule deletions of the highest-level rows first,
then follow with updates starting with lowest-level rows first.

In other words, the sequence is:

 *  delete volume
 *  delete extent
 *  create extent
 *  create volume

We delay volumes to be initialized a bit more to have their dependencies
present when it comes to them being configured.

Signed-off-by: Jan Dvořák <mordae@anilinux.org>