fixing multiple errors in deltatar at once:

fixing multiple errors in deltatar at once:

* naming of the generated backups was incorrect because they were
always named as if they were full backups, even if it was a diff
one. This has been changed both at backup creation and at backup
finding.

Related to that is the backup volume "finder": it was trying to
find volumes with current date which is not what we want, so now
it guesses the date of the volume based on a prefix and a suffix
inside the backup directory.

* changing the ordering of the files in _recursive_walk_dir to
follow an ascending naming sorted Breadth First Search algorithm.
This has been fixed without having to retrieve the full list and
then apply a sort to be efficient, which was something Daniel
applied as a fix and got reverted for memory efficiency.

* fixing index iterator also removing the need to have the full
index in memory and allowing it to be used with the "with"
python statement.

* fixing collate_iterators sorting because when the second index
element was less than the first one, we were not taking into
account that in a BFS algorithm files nearer to the root in depth
go first regardless of the sorting.

* some optimizations and fixes related to the deletion of files
in restore_backup for diff backups have been done