Support exponential cleanup for backups

[Gerrit91]

At the moment we just keep all the backups with a given number of revisions and remove the oldest ones.

When backups are configured very often and with a high number of revisions to keep, this blows up the sizes of the backups quite a lot.

It would be beneficial if there would be less backups that lie longer back in the past.

[robertvolkmann]

A simpler approach to save storage is to generate an additional backup file every hour and retain only versions of db.tar.gz from the last 24 hours. This would result in keeping just 24 backups per day instead of 480.

Example:

db_2007-12-24T19:00:00Z.tar.gz
db_2007-12-24T20:00:00Z.tar.gz
db_2007-12-24T21:00:00Z.tar.gz
db.tar.gz