Support persistence of configuration items modified at runtime via HTTP API.
```
FE:
GET /api/_set_config?key=value&persist=true
BE
POST /api/update_config?key=value&persist=true
```
The modified config will be saved in `fe_custom.conf` or `be_custom.conf`.
And when process starts, it will load `fe.conf/be.conf` first, then `fe_custom.conf/be_custom.conf`.
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`
