[Reviewer: Alex] Log REGISTER and SUBSCRIBE/NOTIFY transactions to SAS correctly

Alex,

Please can you review my fix to #709?  This is simply to add the correct markers to REGISTER and SUBSCRIBE/NOTIFY transactions.  I've done this by adding a `PJUtils::report_sas_to_from_markers` method that works out which markers are appropriate and adds them.  The one exception is in `handlers.cpp` where we don't have a SIP message in our hand (we're handling a Chronos callback).  I've live-tested.  I've also run the UTs, but they don't test SAS behavior.

Thanks,

Matt
What's the impact of allocating a new pool on every HTTP request? If it's cheap then I'll stop worrying, it just seems like a lot of effort for what is essentially some simple string manipulations.