Get rid of struct Time

We just need the milliseconds of current system
time for our needs. This allows to simplify the
API.

No functional change.

src/book.cpp

@@ -340,7 +340,7 @@ namespace {
 
 Book::Book() {
 
-  for (int i = Time::now().msec() % 10000; i > 0; i--)
+  for (int i = Time::now() % 10000; i > 0; i--)
       RKiss.rand<unsigned>(); // Make random number generation less deterministic
 }