discovery: move missing tracking inside the partialdiscovery object This is the final set that we need to track to have a fully up to date information within the object.

discovery: add a `iscomplete` method to the `partialdiscovery` object The method is used by higher level logic to know if the initial discovery question has been answered.

discovery: update undecided from common directly within partialdiscovery The object knows when there are changes in the "common" set, so it is a good place to update the undecided set.