In the past, when we want to modify some BE configs, we have to modify be.conf and then restart BE.
This patch provides a way to modify configs in the type of 'threshold', 'interval', 'enable flag'
when BE is running without restarting it.
You can update a single config once by BE's http API: `be_host:be_http_port/api/update_config?config_name=new_value`
* add authentication to DownloadAction
1. use cluster_id as token;
2. add dir limit, only files in data dir can be accessed.
* enable authentication in DownloadAction by default
