res_pjsip_registrar.c: Fix AOR and pjproject group deadlock.

res_pjsip_registrar.c: Fix AOR and pjproject group deadlock.

One of the patches for ASTERISK_27147 introduced a deadlock regression.
When the connection oriented transport shut down, the code attempted to
remove the associated contact.  However, that same transport had just
requested a registration that we hadn't responded to yet.  Depending
upon timing we could deadlock.

* Made send the REGISTER response after we completed processing the
request contacts and released the AOR lock to avoid the deadlock.

ASTERISK-27391

Change-Id: I89a90f87cb7a02facbafb44c75d8845f93417364