MXS-2196: Always allocate a session

Whenever a client DCB is accepted, a session for it is allocated. This
simplifies the handling of shared data between DCBs by allowing it to be
placed inside the session object. Currently, the data is stashed away in
the client DCB.

server/modules/protocol/CDC/cdc.cc

@@ -144,10 +144,7 @@ static int cdc_read_event(DCB* dcb)
 
             if (auth_val == CDC_STATE_AUTH_OK)
             {
-                /* start a real session */
-                session = session_alloc(dcb->service, dcb);
-
-                if (session != NULL)
+                if (session_start(dcb->session))
                 {
                     protocol->state = CDC_STATE_HANDLE_REQUEST;