Time: use locks again

The Atomic implementation was only partially correct, and was very complex.
Use locks for sake of simplicity, following KISS principle.
Only remaining atomic operation use is time of day speed, because that
really is only read + written.

Also fixes a bug with m_time_conversion_skew only being decremented, never
incremented (Regresion from previous commit).

atomic.h changes:
	* Add GenericAtomic<T> class for non-integral types like floats.

	* Remove some last remainders from atomic.h of the volatile use.