Assert DCB ownership in dcb_drain_writeq

This will cause EPOLLOUT events for maxscaled to trigger the assert
immediately if the buffer was placed into the queue by another worker.

server/core/dcb.cc

@@ -1006,6 +1006,8 @@ static void dcb_log_write_failure(DCB* dcb, GWBUF* queue, int eno)
  */
 int dcb_drain_writeq(DCB* dcb)
 {
+    mxb_assert(dcb->poll.owner == RoutingWorker::get_current());
+
     if (dcb->ssl_read_want_write)
     {
         /** The SSL library needs to write more data */