Before SERVER-52604 it was impossible to log from a header file, so we had to workaround it by introducing some helper logging functions defined in a .cpp file which sole function was to call the logging API. 

We can refactor the following code to directly invoke the logging API from a header and remove helper functions.

• plan_cache_callbacks.h
• plan_ranker.h
• plan_cache_util.h
• query_planner.h