Use memcached tombstone records instead of deletions to avoid write races
Add SAS correlation for authentication timer pops
Markups related to SAS overcorrelation issue
Further updates to SUBSCRIBE/NOTIFY SAS trails
Improve comments, delete a debug log
Review feedback - only use one key
Code review markups
Clean up UTs
Take master PJSIP
Merge pull request #667 from Metaswitch/fix_memcached_ordering

[Reviewer: Alex] Fix memcached ordering and improve SAS traces
Take master cpp-common
Remove rapidxml now it's in cpp-common
Merge pull request #670 from Metaswitch/update_cpp_common

[No review required] Update cpp common