res_pjsip_registrar: blocked threads on reliable transport shutdown take 3

When a contact was removed by the registrar it did not always check to see if
the circumstances involved a monitored reliable transport. For instance, if the
'remove_existing' option was set to 'true' then when existing contacts were
removed due to 'max_contacts' being reached, those existing contacts being
removed did not unregister the transport monitor.

Also, it was possible to add more than one monitor on a reliable transport for
a given aor and contact.

This patch makes it so all contact removals done by the registrar also remove
any associated transport monitors if necessary. It also makes it so duplicate
monitors cannot be added for a given transport.

ASTERISK-28213

Change-Id: I94b06f9026ed177d6adfd538317c784a42c1b17a