[Reviewer Matt] HttpStack / PJSIP threading issues

Matt

Can you review my fix to the HttpStack/PJSIP threading issues.  I think this will fix #745 and it might clean up some of the other weird crashes we have seen since this would in theory result in PJSIP accessing memory that had been subsequently overwritten.

I never found a reliable repro for #745, so I have done some live tests to make sure it hasn't broken anything, but I can't be 100% sure this is the cause of #745.

Mike