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/decoder.hpp

@@ -164,10 +164,18 @@ namespace zmq
 
     private:
 
+        //  Where to store the read data.
         unsigned char *read_pos;
+
+        //  How much data to read before taking next step.
         size_t to_read;
+
+        //  Next step. If set to NULL, it means that associated data stream
+        //  is dead. Note that there can be still data in the process in such
+        //  case.
         step_t next;
 
+        //  The duffer for data to decode.
         size_t bufsize;
         unsigned char *buf;