MXS-2196: Pass client DCB to MXS_PROTOCOL::accept

By doing the actual accepting of the new DCB in the core, the protocol
modules can only do the actual protocol level work. This removes some of
the redundant code that was in the protocol modules.

include/maxscale/protocol.h

@@ -23,11 +23,6 @@
 #include <maxbase/jansson.h>
 #include <maxscale/buffer.h>
 
-#include <memory>
-
-class Listener;
-using SListener = std::shared_ptr<Listener>;
-
 MXS_BEGIN_DECLS
 
 struct DCB;
@@ -94,13 +89,13 @@ typedef struct mxs_protocol
     /**
      * Accept a connection, only for client side protocol modules
      *
-     * @param dcb The listener DCB
+     * @param dcb The client DCB
      *
      * @return 1 on success, 0 on error
      *
      * @note Currently the return value is ignored
      */
-    int32_t (* accept)(const SListener& listener);
+    int32_t (* accept)(DCB* client_dcb);
 
     /**
      * Connect to a server, only for backend side protocol modules