do not flush if Write() failed (#13597)

- Go might reset the internal http.ResponseWriter() to `nil`
  after Write() failure if the go-routine has returned, do not
  flush() such scenarios and avoid spurious flushes() as
  returning handlers always flush.
- fix some racy tests with the console 
- avoid ticker leaks in certain situations

internal/http/server.go

@@ -92,7 +92,6 @@ func (srv *Server) Start(ctx context.Context) (err error) {
 			w.Header().Set("Connection", "close")
 			w.WriteHeader(http.StatusServiceUnavailable)
 			w.Write([]byte(http.ErrServerClosed.Error()))
-			w.(http.Flusher).Flush()
 			return
 		}