Refactor getRRset
That one was getting really hairy and it wouldn't serve it's purpose in the following work. Currently it takes set of types we're interested in and returns all of them, the logic to decide which one to take is left to the caller. This simplifies the method and doesn't hurt the rest much, as the rest needed to know what to ask for anyway. Also, it allows getting NSEC or NSEC3 records in the same database search almost for free.