New configuration to control logging level between INFO and DEBUG

[yuanzhou]

Introduce a new configuration so we can

• Turn on/off the DEBUG logging mode on the fly.
• Set to INFO for PROD deployment, but can still turn on the DEBUG mode for troubleshooting.
• Go through the codebase and decide the proper use of logger.debug() and logger.info().

[yuanzhou]

Addressed via PR #894