display more details in accesslog

[tcodehuber]

now there are only uid and gid related contents, such as belows:

But for hadoop admins, it is very important and convenient to see username and groupname in accesslog files. Look forward to coming true in the future.

[davies]

JuiceFS use uid and gid for permission checking, it will be expensive (may also not accurate) to translate the uid/gid to username for logging.