Refactor the code that generates updates so that up_generate_updates is

Refactor the code that generates updates so that up_generate_updates is
only called in one spot.

rde_generate_updates() gets a enum eval_mode argument to discern
the different cases. peer_generate_update() uses the eval_mode to skip
the update if it is not needed.
While there also add an extra AID check in IMSG_REFRESH case to make sure
the requested AID is actually available for this peer.
OK tb@