res_pjsip_registrar_expire: Fix race condition at shutdown.

res_pjsip_registrar_expire: Fix race condition at shutdown.

When shutting down, the PJSIP sorcery is destroyed. The registrar
expiration module queries the PJSIP sorcery to determine what
to expire. As there was no synchronization between termination
of the expiration thread and the unloading of the module it was
possible for the thread to try to access the PJSIP sorcery after
it had been destroyed.

This change ensures that the thread is shut down before allowing
the module to be considered unloaded.

Change-Id: I69fd239edbaaf160c2d37ae00d3ac06e5596fe8b