Group active backing up images to DATE not SUBVOLUME.

As an experiment I recently switched to btrbk snapshots every 5 minutes from 60 minutes. When I run the nightly "send all my backups to my servers backup disk" it takes a lot longer of course, as estimated.

I have four subvolumes that are being backed up: root, home, boot and data. On root is my OS, on home is /home, on boot I ```rsync -avr --delete``` my /boot (ext2) drive before every backup and on /data is my Music and Movie collection.

So with my backups taking a lot longer now to the external file server and my data being seperated in this way, I noticed that btrbk is syncing snapshots first by name and only then by date, like this:

root_at_time_1
root_at_time_2
root_at_time_3
root_at_time_4
home_at_time_1
home_at_time_2
home_at_time_3
home_at_time_4
boot_at_time_1
boot_at_time_2
boot_at_time_3
boot_at_time_4
data_at_time_1
data_at_time_2
data_at_time_3
data_at_time_4

It would be better for consistency if it was in this order:

root_at_time_1
home_at_time_1
boot_at_time_1
data_at_time_1
root_at_time_2
home_at_time_2
boot_at_time_2
data_at_time_2
root_at_time_3
home_at_time_3
boot_at_time_3
data_at_time_3
root_at_time_4
home_at_time_4
boot_at_time_4
data_at_time_4

The issue describes btrbk's backup transfer ordering across the root, home, boot, and data subvolumes; start at the entry point that selects snapshots for remote backup and inspect how they are grouped. Done means transfers are grouped by snapshot date across subvolumes in the requested sequence, with the existing test suite run afterward.

Written by the indexing model from the issue text.