PGM transport reconciled with subscription forwarding

As PGM is not capable of passing subscriptions upstream,
subscriptions are ignored at sub side and engine subscribes
for all messages on pub side.

Signed-off-by: Martin Sustrik <sustrik@250bpm.com>

src/pgm_receiver.hpp

@@ -64,6 +64,10 @@ namespace zmq
 
     private:
 
+        //  PGM is not able to move subscriptions upstream. Thus, drop all
+        //  the pending subscriptions.
+        void drop_subscriptions ();
+
         //  RX timeout timer ID.
         enum {rx_timer_id = 0xa1};