Rewrite time measurement code

Introduce and use a new Time class designed after
QTime, from Qt framework. Should be a more clear and
self documented code.

As an added benefit we now use 64 bits internally to get
millisecs from system time. This avoids to wrap around
to 0 every 2^32 milliseconds, which is 49.71 days.

No functional change.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>