mutexd

I once encountered this issue. After apply the patch and add-module of nginx_upstream_check_module, be sure to add compile option **-DNGX_HTTP_UPSTREAM_CHECK**. Otherwise the patched code won't be compiled. Hope this help....

@dbidwell94 I see that you're using `futures::executor:block_on` in the `drop`. The catch is that `futures::executor::block_on` would block the current caller until the future has completed. In the test, there is...