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>

src/book.cpp

@@ -342,7 +342,7 @@ namespace {
 
 Book::Book() : size(0) {
 
-  for (int i = abs(system_time() % 10000); i > 0; i--)
+  for (int i = Time::current_time().msec() % 10000; i > 0; i--)
       RKiss.rand<unsigned>(); // Make random number generation less deterministic
 }