res_pjsip: Fix crash when reloading certain configurations.

Certain options available that specify a SIP URI perform validation
on the provided URI using the PJSIP URI parser. This operation
requires that the thread executing it be registered with the PJLIB
library. During reloads this was done on a thread which was NOT
registered with it.

This fixes the problem by creating a task which reloads the
configuration on a PJSIP thread.

(closes issue ASTERISK-22923)
Reported by: Anthony Messina
........

Merged revisions 403179 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@403180 65c4cc65-6c06-0410-ace0-fbb531ad65f3