DNS Update Misc

71436da needs a closer look since I just took a guess and the tests passed.

104b206 makes `RemoveRRset()` work as intended, though I think those update functions shouldn't actually behave the way they do. It makes sense for `SetQuestion()` to clear the question section, but I don't think the prerequisite and update helpers should clear they're respective sections as stacking a series of prerequisites and changes is likely common.