Babel: Rework handling of retractions

An update with wildcard AE and infinite metric should be treated as a
global retraction of all prefixes announced by that neighbour, per
section 4.4.9 of the RFC. In addition, router ID and seqno in retraction
updates should be ignored. This reworks the handling of retractions and
adjusts the parser to handle all this correctly.

Signed-off-by: Toke Høiland-Jørgensen <toke@toke.dk>