[Positioner] Timestamp::elapsed(): use current time

Timestamp::elapsed() now calls Timestamp::get_current_time() instead of
comparing to now().

owlps-positioner/src/timestamp.cc

@@ -126,11 +126,7 @@ bool Timestamp::now()
  */
 Timestamp Timestamp::elapsed() const
 {
-  Timestamp now ;
-  if (! now.now())
-    return Timestamp() ;
-
-  return elapsed(now) ;
+  return elapsed(get_current_time()) ;
 }