Creation time TTL based snapshot management to replace dynamic rule based deletion

[hirak99]

Note: This issue depends on #6 being done first.

Currently which snapshots will be deleted on schedule is determined dynamically, based on the rules given the config rules (e.g. keep_hourly = ..., keep_daily = ..., etc.).

We should allow this to be determined at creation time instead, and assign a TTL. The main advantage will be

The feature must maintain back-compatibility. I.e. should gracefully handle existing snaps, also provide the option to follow current logic for those who want it. We can add a new config item, enable_ttl_based_cleanup = true. If set to true, all new scheduled snaps will get assigned a TTL based on the keep_... rules.

[hirak99]

The algorithm for determining what the TTL should be on scheduled snapshot creation can get tricky.

Here is a write-up on required properties, and a proposed algorithm - https://github.com/hirak99/yabsnap/wiki/TTL%E2%80%90Based-Scheduled-Snapshots

[hirak99]

I have been testing the implementation of 42, and will continue to test it.

Thoughts -

• So far I like the transparency in terms of TTL with scheduled snaps - I feel it's a game changer to be able to know exactly how long a snap will last.
• The main drawback with this approach, is that if you don't use the computer for a long time - longer than your keep's - then all your scheduled snaps can expire. It should be an edge case if the installation is active. Also, there is still the option to set enable_scheduled_ttl = False to switch back to older behavior, for any config.

[hirak99]

Added a new release with the default behavior updated to include TTL for scheduled snaps.

What this change means:

• All scheduled snaps will now have TTL by default.
• Scheduled snaps will honor retainment policies with TTL, which will be determined at the time of creation and be forward-looking.
• Users will have visibility into exactly which snap will be deleted and when, as TTL will be enforced and not deleted prematurely.
• Users can manually delete any snap, including those with TTL.
• Users can revert to the old behavior by setting enable_scheduled_ttl = False in the config(s).

Note: This change may cause temporary confusion, as both TTL and non-TTL snaps will coexist for a period. However, the new behavior is more transparent, and all changes are reversible. The bug will remain open to track any issues that may arise.