### _Why are the changes needed?_
It can help to know the backend pressure if the exec pool is full.
### _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.readthedocs.io/en/master/develop_tools/testing.html#running-tests) locally before make a pull request
Closes#4344 from turboFei/wait_queue.
Closes#4344
161e3808a [fwang12] nit
6d122e238 [fwang12] save
55a4b499d [fwang12] version
668ff8bfe [fwang12] save
9f56b98a8 [fwang12] save
a401771ec [fwang12] wait
Authored-by: fwang12 <fwang12@ebay.com>
Signed-off-by: fwang12 <fwang12@ebay.com>