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/maxscaled/maxscaled.cc

@@ -352,7 +352,7 @@ static int maxscaled_accept(DCB* listener)
     socklen_t len = sizeof(struct ucred);
     struct ucred ucred;
 
-    while ((client_dcb = dcb_accept(listener)) != NULL)
+    while ((client_dcb = dcb_accept(listener->listener)) != NULL)
     {
         MAXSCALED* maxscaled_protocol = (MAXSCALED*)calloc(1, sizeof(MAXSCALED));