logging: Also set log level of logger handlers (#7576)
After #7526 the default logger passes logs to a StreamHandler, which has
its own log level. Changing the log level of the logger alone does not
take effect in such case.
Update the log level of all handlers when changing the parent logger's.
Signed-off-by: Junjie Mao <junjie.mao@linux.alibaba.com>
