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/xpub.cpp

@@ -169,3 +169,15 @@ void zmq::xpub_t::send_unsubscription (unsigned char *data_, size_t size_,
     }
 }
 
+zmq::xpub_session_t::xpub_session_t (io_thread_t *io_thread_, bool connect_,
+      socket_base_t *socket_, const options_t &options_,
+      const char *protocol_, const char *address_) :
+    session_base_t (io_thread_, connect_, socket_, options_, protocol_,
+        address_)
+{
+}
+
+zmq::xpub_session_t::~xpub_session_t ()
+{
+}
+