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`.