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### _Why are the changes needed?_
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For detail, please go to _Originally posted by yaooqinn in https://github.com/apache/incubator-kyuubi/issues/962#issuecomment-905116261_
### _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
```
./bin/beeline -u 'jdbc:hive2://localhost:10009/;?kyuubi.engine.pool.enabled=true;kyuubi.engine.pool.size=3;'
```
Closes#986 from timothy65535/ky-962-v2.
Closes#962
b79f8778 [timothy65535] fix test name
9a435607 [timothy65535] fix style
abc746b8 [timothy65535] update tests
71b6933d [timothy65535] revert changes
018dee8b [timothy65535] revert changes
e3492934 [timothy65535] revert changes
67fc8fd9 [timothy65535] improve threshold
fa52acb2 [timothy65535] update pool threshold
d8b5977d [timothy65535] remove dup get
7f39b8d7 [timothy65535] fix test error
4fdf8d76 [timothy65535] update thread pool size to 1 -> threshold
2477348a [timothy65535] rich engine pool tests
8eee427f [timothy65535] fix update error msg
104be5ea [timothy65535] fix error tests
8c2d1441 [timothy65535] fix doc
ddd0eda9 [timothy65535] improve doc & tests
83d7c5cf [timothy65535] update doc
4b6c7ee3 [timothy65535] improve subdomain
169d9d06 [timothy65535] update todo
1424e565 [timothy65535] increase pool size range
3523cd6c [timothy65535] update todo
2cbeafb2 [timothy65535] remove enable option
424b5364 [timothy65535] revert ServiceDiscovery
6fc1c510 [timothy65535] remove random policy
9feef8f3 [timothy65535] [KYUUBI #962] Support engine pool feature
Authored-by: timothy65535 <timothy65535@163.com>
Signed-off-by: Kent Yao <yao@apache.org>
acb4f29906