Option to soft-delete a completed saga data

[SeanFeldman]

Currently

When a saga instance is no longer needed it can be completed using the MarkAsComplete() API. This tells the saga infrastructure that the instance is no longer needed and can be cleaned up.

This means that when the saga is completed, there's no way to audit the state of the completed processes as data is deleted. It would help to have an option to "soft-delete" sagas upon completion.

[kbaley]

@SeanFeldman Agreed, this would be a nice feature. We'll include this in an upcoming product enhancement.