[Reviewer: Graeme] Craft logging Enhanced Node Troubleshooting 

Added PDLog infrastructure (definition in new header, craft_dcea.h) and PDLogs for http and sip.
I'm puzzled why we need to solve the variable number of parameters issue by templating rather than just using va_list args as we do in the Log::_write method in cpp-common (https://github.com/Metaswitch/cpp-common/blob/master/src/log.cpp#L97).