Session class separated into socket-type-specific sessions

This is a preliminary patch allowing for socket-type-specific
functionality in the I/O thread. For example, message format
can be checked asynchronously and misbehaved connections dropped
straight away.

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

src/req.cpp

@@ -146,4 +146,15 @@ bool zmq::req_t::xhas_out ()
     return xreq_t::xhas_out ();
 }
 
+zmq::req_session_t::req_session_t (io_thread_t *io_thread_, bool connect_,
+      socket_base_t *socket_, const options_t &options_,
+      const char *protocol_, const char *address_) :
+    xreq_session_t (io_thread_, connect_, socket_, options_, protocol_,
+        address_)
+{
+}
+
+zmq::req_session_t::~req_session_t ()
+{
+}