QUIC: asynchronous shutdown. Previously, when cleaning up a QUIC stream in shutdown mode, ngx_quic_shutdown_quic() was called, which could close the QUIC connection right away. This could be a problem if the connection was referenced up the stack. For example, this could happen in ngx_quic_init_streams(), ngx_quic_close_streams(), ngx_quic_create_client_stream() etc. With a typical HTTP/3 client the issue is unlikely because of HTTP/3 uni streams which need a posted event to close. In this case QUIC connection cannot be closed right away. Now QUIC connection read event is posted and it will shut down the connection asynchronously.