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.
Your questions are answered in the comment above the function, and in the existence of the enum which enumerates the four states.  It only takes a few seconds to confirm that this code does exactly what the comment says it does.