Concurrent backup within single hosts

[ewxrjk]

#18 enabled concurrent backup of distinct hosts to distinct devices.

This issue is for concurrent backup of distinct volumes within a single host to distinct devices. This still makes a reasonable amount of sense if the volumes are on separate storage, or on shared storage that is much faster than the backup device (e.g. volumes on SSD, backups on USB spinning rust).

With that in mind it should be under operator control whether two volumes can be backed up concurrently.

It is probably blocked by #17.