Changes to stop keepalives on unregisters

Observed cases where keepalives from `nat_traversal` continued after unregister were traced back to a few lines of code which prevented stopping keepalives on unregister events.
(cherry picked from commit f4ed457bc10d6ddfcd6dc77dcc9e460e767bdc4b)

(cherry picked from commit 8187a92d48abe24643f8ea2f1b0988857f0ccb1b)