make purge more intelligent

[Andrew Garner]

Instead of failing a backup due to lack of disk space, it probably makes sense to purge old backups first. This should be a configurable option but the general approach could be:

1) estimate the space we probably will need for a backup
2) if we don't have enough space, start killing older backups until we have space
3) Still try anyway, and let a watchdog process abort holland if we really don't have enough space

This can tie into more intelligent estimation methods like linear-extrapolation or just following the general trend of compression ratios, or looking at the size of the previous backups so we can achieve more accurate "required space" estimation.

[Andrew Garner (muzazzi)]

Added 'purge-on-demand' option to [holland:backup]. If set, this purges the minimum number of backups that would free enough space for the estimated space required for the current in-progress backup. If the sizes of all the old backups is not sufficient to allow the next backup to proceed the backup will still fail and no old backups will be purged.

Fix committed here: https://github.com/holland-backup/holland/commit/fbb1252621b2e293a1e960673bdaa70ac329685b