Template abstract_clock on Clock:

The abstract_clock is now templated on a type meeting the requirements of
the Clock concept. It inherits the nested types of the Clock on which it
is based. This resolves a problem with the original design which broke the
type-safety of time_point from different abstract clocks.

Reviewers: @howardhinnant, @scottschurr 
Oh, I just noticed that `now()` should probably be marked `override`.