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/core/routingworker.cc

@@ -412,6 +412,7 @@ bool RoutingWorker::add_shared_fd(int fd, uint32_t events, MXB_POLL_DATA* pData)
     ev.events = events;
     ev.data.ptr = pData;
 
+    // The main worker takes ownership of all shared fds
     pData->owner = RoutingWorker::get(RoutingWorker::MAIN);
 
     if (epoll_ctl(this_unit.epoll_listener_fd, EPOLL_CTL_ADD, fd, &ev) != 0)