Simplification of basic_seconds_clock.

This eliminates an unnecessary vector of pointers from the basic_seconds_clock implementation.  This may allow us to remove the vc++ hack of stopping the clock at the end of main.
Can you make this into a Javadoc comment please?  The plan is to start to use Doxygen more often to generate external documentation:  https://www.youtube.com/watch?v=iq8BB5YOyCI&src_vid=nz1cEO01LLc&feature=iv&annotation_id=annotation_2008016577