10/6/2014, updated 9/13/2016
Singleton class X must now do the following:
1) using CRTP, derive publicly from utility::SingletonBase< X >
2) define a private, static function X * create_singleton_instance();
3) declare class utility::SingletonBase< X > to be a friend so that it
(and no one else) can call create_singleton_instance()
The SingletonBase class defines the static X * get_instance() function and handles
the instantiation of the instance in a thread-safe manner. The instance_ pointer
belongs to the base class.
Singletons, with certain explicit exceptions (the JobDistributor, the ResourceManager) should not hold job-specific data.