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>