Refactor the transport interface

- give transport Initialize() method with access to device config.
 - avoid using global context in the transport.
 - simplify shutdown procedure (no need for extra thread).

fairmq/runFairMQDevice.h

@@ -52,6 +52,11 @@ int main(int argc, char** argv)
         config.ParseAll(argc, argv);
 
         std::unique_ptr<FairMQDevice> device(getDevice(config));
+        if (!device)
+        {
+            LOG(ERROR) << "getDevice(): no valid device provided. Exiting.";
+            return 1;
+        }
         int result = runStateMachine(*device, config);
 
         if (result > 0)