Added a abstract placeholder for callbacks in asio (for now); which has
two subclasses; one takes a DNSServer* and calls resume() on it when the
runningquery finished its lookup (or fails), the other one is provided a
callback as defined in isc::nsas::ResolverInterface.

These are more objects that are newed while running and delete
themselves, and they may not even be necessary, but for now this makes
it work while doing the least amount of changes in other parts.

RunnigQuery now takes one of these objects instead of a DNSServer*, and
the objects decide what should be done once it is ready.

Things to do:
- remove the temporary MyCallback class from resolver (perhaps move it
  to a unittest)
- move ResolverInterface out of nsas, as well as its Callback thingy,
  and merge it with the new callback holders
- general cleanup and tests (although, as with current resolver tests,
  need framework to fake outside query handling)