Support "optional" and "mandatory" reviewers

Support for workflows where I’d like to set a “mandatory” and “optional” reviewers. Review doesn’t get closed unless all mandatory (or one?) reviewers have finished. Additionaly, I may not wish to wait for optional reviewers. “Due Time” could be used to close the review if workflow is set that way. I’m using it now only to signal my wish. However sometimes I’d like to be more strict about it.