Fix issue #370

The patch extends the internal session's API with the reset method.
This method is used to reset a session's state so that it can
handle a new connection.

src/req.cpp

@@ -178,3 +178,8 @@ int zmq::req_session_t::write (msg_t *msg_)
     return -1;
 }
 
+void zmq::req_session_t::reset ()
+{
+    session_base_t::reset ();
+    state = identity;
+}