[Reviewer Alex] UASTsx leak if Sproutlet generates ACK locally

Alex

Can you review my fix to #822.  This is the leak issue Ellie found testing some REFER cases.  I've tested by adding a B2BUA test case to the sproutlet proxy tests.  I also added a test case for chained Sproutlets while I was trying to work out what the exact problem scenario was, and it's worth keeping this in.

There's also a one line code layout fix in registration_utils.cpp.

Mike
Better to use `pjsip_method_set`