Problem: duplicated code & inconsistent behaviour between

mechanisms

Solution: uniformly require a ZAP domain to be set to activate ZAP
handling, clarify comment on Stonehouse pattern

src/null_mechanism.hpp

@@ -55,8 +55,6 @@ namespace zmq
         virtual int zap_msg_available ();
         virtual status_t status () const;
 
-        bool zap_required () const;
-
     private:
 
         bool ready_command_sent;