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`
1. Fix error class in start_fe.sh and start_broker.sh.
2. Add log4j2.xml in fe/src/test/resources/ to run fe ut without log4j warnings.
3. Reduce the test file size in be ut.
