Problem: zmq_poller only signals one event

Solution: zmq_poller_wait_all signals all events

allows signaling multiple events with one call to zmq_poller_wait_all
rather than emitting only one event.

this prepares for zmq_poll being based on zmq_poller,
which requires events for all sockets rather than just one.

src/socket_poller.hpp

@@ -75,6 +75,8 @@ namespace zmq
 
         int wait (event_t *event, long timeout);
 
+        inline int size (void) { return items.size (); };
+
         //  Return false if object is not a socket.
         bool check_tag ();