res_pjsip_outbound_registration: Ensure URI validation happens in a pjlib thread.

This change moves outbound registration URI validation into the task executed
within a pjlib thread.

Reported by: Andrew Nagy


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/12@404725 f38db490-d61c-443f-a65b-d21fe96a405b