Check message syntax in REQ asynchronously

This patch adds support for checking messages as they arrive
(as opposed to when they are recv'd by the user) and drop
the connection if they are malformed.

It also uses this new feature to check for validity of inbound
messages in REQ socket.

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

src/session_base.hpp

@@ -48,8 +48,8 @@ namespace zmq
         void attach_pipe (class pipe_t *pipe_);
 
         //  Following functions are the interface exposed towards the engine.
-        bool read (msg_t *msg_);
-        bool write (msg_t *msg_);
+        virtual int read (msg_t *msg_);
+        virtual int write (msg_t *msg_);
         void flush ();
         void detach ();