prune mark: correctly keep track of already included backups

prune mark: correctly keep track of already included backups

This needs to happen in a separate loop, because some time intervals are not
subsets of others, i.e. weeks and months. Previously, with a daily backup
schedule, having:
* a backup on Sun, 06 Dec 2020 kept by keep-daily
* a backup on Sun, 29 Nov 2020 kept by keep-weekly
would lead to the backup on Mon, 30 Nov 2020 to be selected for keep-monthly,
because the iteration did not yet reach the backup on Sun, 29 Nov 2020 that
would mark November as being covered.

Signed-off-by: Fabian Ebner <f.ebner@proxmox.com>