res_pjsip_outbound_registration.c: Add a serializer shutdown group.

The client_state objects contain a serializer used to send the outbound
REGISTER messages.  Once all those message transactions are complete then
the module can shutdown.

ASTERISK-24907 #close
Reported by: Kevin Harwell

Change-Id: Ibb2fe558f98190f2a06da830e0fadfa25516f547