In ec2 centOs testing, when `user` is not provided. `AlluxioFuseFileSystem.chown()` will provide a really big number as the `uid: 4294967295`. This uid does not exist in unix and `chown()` will not work.
Need to investigate what uid will be provided to `AlluxioFuseFileSystem.chown()` when user is not provided in `chown and chgrp` in different environment(different linux/mac).
And need to support change group without change user or document this limitation.
in macOS, if user name does not exist, ls shows 4294967295 as user name.
According to https://stackoverflow.com/questions/22914627/some-uids-in-proc-pid-loginuid-are-strange, 4294967295 is just (unsigned long) -1. -1 means that loginuid was not set
In AlluxioFuseFileSystem.chown(), if uid is given as -1 or 4294967295, we can know that user do not want to set user name(uid) and we do not change user name of the file.
In this way, we could support chown :groupName and chgrp.