Add option to remove (oldest) snapshot when space is low

[RlndVt]

Unless I'm missing something, btrbk has no (configurable) check as to the free space on the host nor the target. An option to remove the oldest snapshot if the free-space is lower than say 2% would be a nice addition.

[ghost]

This is a good idea. Some thoughts need to go into some sensible defaults, like preserving at least one snapshot on the source and target.

[zwimer]

This would be amazing! That way I wouldn't have to go in and manually delete old backups every time my disk fills up.