MXS-2196: Pass Listener into dcb_accept

Modified the functions to use a listener instead of a DCB in the accepting
process. This removes some of the dependenices that the listeners have on
the DCB system.

server/modules/protocol/MySQL/mariadbclient/mysql_client.cc

@@ -1397,7 +1397,7 @@ int gw_MySQLAccept(DCB* listener)
 {
     DCB* client_dcb;
 
-    while ((client_dcb = dcb_accept(listener)) != NULL)
+    while ((client_dcb = dcb_accept(listener->listener)) != NULL)
     {
         gw_process_one_new_client(client_dcb);
     }   /**< while client_dcb != NULL */