DestructibleSingletonHolder is a little utility that helps in building
singletons that can be created and destroyed multiple times during the lifetime
of a program. The class holds a unique_ptr to the actual singleton
implementation and an instance of a factory for allocating that unique_ptr and
initializing the singleton.

The class is accompanied by a function template that stores an instance of the
class in a static variable, which provides for the singleton. This function must
always be called with the factory function, as it is templated on it. This makes
it possible to keep the singleton private to a class by using a private static
member function as the factory.