QUIC: revised ngx_quic_handle_datagram() error codes. The NGX_DECLINED is replaced with NGX_DONE to match closer to return code of ngx_quic_handle_packet() and ngx_quic_close_connection() rc argument. The ngx_quic_close_connection() rc code is used only when quic connection exists, thus anything goes if qc == NULL. The ngx_quic_handle_datagram() does not return NG_OK in cases when quic connection is not yet created.