res_pjsip: Fix prune_on_boot to remove only contacts for the host.

res_pjsip: Fix prune_on_boot to remove only contacts for the host.

* Check that the contact's reg_server matches the host's name before
deleting any prune_on_boot contacts.  We don't want to delete reliable
transport contacts made with other servers if the ps_contacts database
table is shared with other servers.

Thanks to Ross Beer for pointing out that the original prune logic would
delete reliable transport contacts from other servers.

ASTERISK-27147

Change-Id: I8e439d0d1c266ffdfd7b73d1e5e466180a689bd0