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.
src/ripple/unity/app.cpp, which contains main(), calls a private function.  This call is only necessary on vc2013, and even that is in doubt.  A simple test will confirm one way or the other, and a comment at the call in main() indicates this.  In the meantime, the compiler ensures that this private function is not called elsewhere in rippled by making main a friend.  In order to make main a friend, this code needs to see the main prototype.