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### _Why are the changes needed?_
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As we now fully support GROUP share level, the session user and the system user can be different, so we now need to add UDF session_user.
Also, we fix the system_user to always return the `sparkUser`
### _How was this patch tested?_
- [x] Add some test cases that check the changes thoroughly including negative and positive cases if possible
- [ ] Add screenshots for manual tests if appropriate
- [x] [Run test](https://kyuubi.readthedocs.io/en/latest/develop_tools/testing.html#running-tests) locally before make a pull request
Closes#1274 from yaooqinn/660.
Closes#660
c8ed1f77 [Kent Yao] dead code
20c18640 [Kent Yao] nit
4ab1476c [Kent Yao] nit
081f32f2 [Kent Yao] nit
e332ca78 [Kent Yao] Merge branch 'master' into 660
135b058f [Kent Yao] ci
646ddeec [Kent Yao] [KYUUBI #660] Add UDF session_user
Authored-by: Kent Yao <yao@apache.org>
Signed-off-by: Kent Yao <yao@apache.org>
7b3442a709