This patch prepares the way for having a separate StickyTimeDuration class
by factoring TimeDuration into a templated base class: BaseTimeDuration.
BaseTimeDuration takes a templated parameter, ValueCalculator, which is a helper
object that defines how various arithmetic operations are performed on its
mValue member (an int64_t count of ticks).
This patch does not actually define or use the ValueCalculator parameter yet but
simply performs the renaming and templatization.
With regards to the templatization, arithmetic operators are defined to take
objects with the same ValueCalculator template parameter (so that we don't, for
example, apply non-safe arithmetic to a StickyTimeDuration).
However, comparison operators are defined to also operate on objects with
a different ValueCalculator template parameter since comparison should be
independent of the type of arithmetic used.
Likewise, the constructor and assignment operator are defined to operate on
objects with a different ValueCalculator template parameter so that objects can
be converted from TimeDuration to StickyTimeDuration and vice-versa.
The constructor is marked as explicit, however, so that we don't silently
convert a StickyTimeDuration to a TimeDuration and unwittingly apply
non-safe arithmetic to a StickyTimeDuration.
TimeDuration is defined as a specialization of BaseTimeDuration that uses
TimeDurationValueCalculator as its ValueCalculator type.
TimeDurationValueCalculator is filled-in in a subsequent patch.
In order to have different templated versions of TimeDuration we first split out
the platform-specific code so that this code doesn't need to concern itself with
templates (and because putting template code in .cpp files is messy).
