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.

server/modules/routing/binlogrouter/blr_master.cc

@@ -226,11 +226,7 @@ static void blr_start_master(void* data)
         return;
     }
 
-    /**
-     * 'client' is the fake DCB that emulates a client session:
-     * we need to set the poll.thread.id for the "dummy client"
-     */
-    router->client->owner = mxs_rworker_get_current();
+    mxb_assert(router->client->owner == mxs_rworker_get_current());
 
     /* Connect to configured master server */
     if ((router->master = dcb_connect(router->service->dbref->server,