res_pjsip: Fix deadlock on reliable transport shutdown.

A deadlock can happen when the PJSIP monitor thread is shutting down a
connection oriented transport (TCP/TLS) used by a subscription at the same
time as another thread tries to send something for that subscription.  The
deadlock is between the pjsip monitor thread attempting to get the dialog
lock and another thread sending something for that dialog when it tries to
get the transport manager lock.

* res_pjsip_pubsub.c: Avoid the deadlock by pushing the subscription
removal to the subscription serializer.

* res_pjsip_registrar.c: Pushed off incoming registration contact removals
to a default serializer as a precaution.  Removing the contacts involves
sorcery access which in this case will involve database access.  Depending
upon the setup, the database may not be on the same machine and could take
awhile.  We don't want to hold up the pjsip monitor thread with
potentially long access times.

ASTERISK-27706

Change-Id: I56b647aea565f24dba33e9e5ebeed4cd3f31f8c4

res/res_pjsip_outbound_registration.c

@@ -833,6 +833,8 @@ static int reregister_immediately_cb(void *obj)
  *
  * \param obj What is needed to initiate a reregister attempt.
  *
+ * \note Normally executed by the pjsip monitor thread.
+ *
  * \return Nothing
  */
 static void registration_transport_shutdown_cb(void *obj)