juicefs support dynamic change log level to debug better

[liujiangang01]

What would you like to be added:
Now, when we want to debug, we must restart juicefs process to specify log level, it will result in business interrupting, I wish juicefs can change log level when running like cephfs

Why is this needed:
Now debug by changing log level will result in business interrupting

[davies]

Once we can restart juicefs gracefully without interrupting applications, we can change the logging level easily.

We will support graceful restart in 1.2 release.

[zhijian-pro]

done #4392