Move DCB owner selection into Listener

The code that selects which worker to assign the DCB to is now completely
in the Listener class. This removes the need to change the ownership of a
DCB after it has been allocated.

include/maxscale/listener.hh

@@ -45,7 +45,8 @@ public:
     {
         UNIX_SOCKET,    // UNIX domain socket shared between workers
         SHARED_TCP,     // TCP listening socket shared between workers
-        UNIQUE_TCP      // Unique TCP listening socket for each worker
+        UNIQUE_TCP,     // Unique TCP listening socket for each worker
+        MAIN_WORKER,    // Listener that always moves the execution to the main worker
     };
 
     /**