Fix #88: Timeout handling. (#91)

fix timeout handling

* Despite what is documented in sd_bus_get_timeout(3), the timeout
  returned is actually an absolute time point of Linux's CLOCK_MONOTONIC
  clock. Hence, we first have to subtract the current time from the
  timeout in order to get a relative time that can be passed to poll.

* For async call timeouts to reliably work, we need a way to notify the
  event loop of a connection that is currently blocked waiting in poll.
  I.e. assume the event loop thread entered poll with a timeout set to
  T1. Afterwards, the main thread starts an async call C with a timeout
  T2 < T1. In order for C to be canceled after its timeout T1 has
  elapsed, we have to be able to notify the event loop so that it can
  update its poll data.

Co-authored-by: Urs Ritzmann <ursritzmann@protonmail.ch>
Co-authored-by: Lukasz Marcul <lukasz.marcul@onemeter.com>

src/MessageUtils.h

@@ -57,6 +57,12 @@ namespace sdbus
         {
             return _Msg{msg, sdbus, adopt_message};
         }
+
+        template<typename _Msg>
+        static _Msg create(void *msg, internal::ISdBus* sdbus, const internal::IConnection* connection, adopt_message_t)
+        {
+            return _Msg{msg, sdbus, connection, adopt_message};
+        }
     };
 
     PlainMessage createPlainMessage();