### _Why are the changes needed?_
Fix https://github.com/apache/incubator-kyuubi/issues/3512
Currently, Kyuubi EventBus uses `scala.concurrent.ExecutionContext.Implicits.global` to execute async event handler. Generally, it's discouraged to use that global ec, instead, we should create dedicated ec for each workload, this pr aims to use dedicated ExecutionContext for EventBus async execution
### _How was this patch tested?_
- [ ] 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.apache.org/docs/latest/develop_tools/testing.html#running-tests) locally before make a pull request
Closes#3524 from Yikf/ec.
Closes#3512
9757372d [yikf] Use dedicated ExecutionContext for EventBus async execution
Authored-by: yikf <yikaifei1@gmail.com>
Signed-off-by: Cheng Pan <chengpan@apache.org>
12a85fe337