Add a Task 'implementation version', to allow Task implementers to invalidate artifacts

[stuhood]

The implementation version would be a (string) property of a Task that a Task implementer could use to invalidate all artifacts for the Task.

Additionally, in the case of Task.incremental==True, the implementation version should be used to skip incrementalism. The easiest way to do this would likely be to include the current implementation version in the results_dir path, and to not persist it in the build invalidator.

[digwanderlust]

I should be able to add an impl-version attribute to task objects and add the task and all its parents impl-version to the fingerprint. I will also need to update the invalidation logic so that it keeps track of a task version. This will enable us to draw not pull in previous incremental work done from a different version.

[digwanderlust]

https://rbcommons.com/s/twitter/r/3331/

[digwanderlust]

merged

[stuhood]

Thanks Matt!

[digwanderlust]

After speaking with @patricklaw there is some preference to using explicit calls to super in individual task classes. I will update the code and resubmit an implementation that doesn't use descriptors and explicit calls to mro.

[digwanderlust]

https://rbcommons.com/s/twitter/r/3351/

[digwanderlust]

resolved with https://rbcommons.com/s/twitter/r/3381/