A0-4155: Unit saving pipeline

[timorleph]

Make the unit saving into a pipeline, to simplify the semantics of units being officially added to the Dag.

[github-actions]

Please make sure the following happened

• Appropriate tests created
• Infrastructure updated accordingly
• Updated existing documentation
• New documentation created
• Version bumped if breaking changes

[woocash2]

Including backup save in the pipeline is overall a good idea, although I don't support the idea of saving (unit, parents) in the backup. If you want the DagUnit to go through the backup, no problem with that, just when doing backup save, only save the inner unit, and pass the whole DagUnit further in the pipeline.

If there's some hidden idea behind saving the whole DagUnit, please explain.

[kostekIV]

lgtm, nice change in the add_unit function but a little risky :P

[balqaasem]

lgtm, nice change in the add_unit function but a little risky :P

What makes it a little risky?

[timorleph]

What makes it a little risky?

I think @kostekIV means that it will no longer remove the units from the cache immediately after they are being processed by the dag component, which was a nice property to have (no need to remember to call finished_processing outside of dag). It's not really that risky, since the cache cannot create different outputs if it's not cleaned, worst case scenario it would use a tad more memory.