pf: mark removed connections within a multihome association as shutting down

Parse IP removal in ASCONF chunks, find the affected state(s) and mark
them as shutting down. This will cause them to time out according to
PFTM_TCP_CLOSING timeouts, rather than waiting for the established
session timeout.

MFC after: 3 weeks
Sponsored by: Orange Business Services

(cherry picked from commit 4d3af82f78961180149e8a579b481dc382f76eef)