Simplify FSTime() - always use an integer ns internal representation.

Simplify FSTime() - always use an integer ns internal representation.

On platforms without ns timestamps, just convert the python double
approximation to integer nanoseconds (internally).  That simplifies
the code, leaving only one internal representation, and since the
timestamps are actually recorded as timespecs, the double values were
going to be cycled through a nanosecond resolution integral
representation eventually anyway.