Notify ChannelQueue that the response router thread is finishing

[CiprianAnton]

In case the response_router encounters an error, the thread will exit and there is no code to check if it's still alive. This would mean waiting for messages that never arrives.

Added a simple flag based notification mechanism where the route_responses will notify channels that the thread is exiting.

Also because the route_responses thread depends on channels to be created, it is important to make sure we don't start this thread before starting channels.

[codecov-commenter]

Codecov Report

Merging #896 (a717f84) into main (4f1e09e) will increase coverage by 0.92%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #896      +/-   ##
==========================================
+ Coverage   70.52%   71.44%   +0.92%     
==========================================
  Files          65       65              
  Lines        7698     7705       +7     
  Branches     1287     1289       +2     
==========================================
+ Hits         5429     5505      +76     
+ Misses       1879     1805      -74     
- Partials      390      395       +5     

Impacted Files	Coverage Δ
jupyter_server/gateway/managers.py	81.21% <100.00%> (+17.69%)	⬆️
jupyter_server/services/kernels/kernelmanager.py	78.64% <0.00%> (-1.95%)	⬇️
jupyter_server/gateway/gateway_client.py	78.02% <0.00%> (+2.74%)	⬆️

---

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[kevin-bates]

@CiprianAnton - these changes look good - thank you. Tested the changes via elyra/papermill/nbclient using a k8s-enabled enterprise gateway.

[blink1073]

@meeseeksdev please backport to 1.x

[lumberbot-app]

Owee, I'm MrMeeseeks, Look at me.

There seem to be a conflict, please backport manually. Here are approximate instructions:

1. Checkout backport branch and update it.

git checkout 1.x
git pull

2. Cherry pick the first parent branch of the this PR on top of the older branch:

git cherry-pick -x -m1 ee40dbc985e45f5c2699626e0f8f350b7f4989ac

3. You will likely have some merge/cherry-pick conflict here, fix them and commit:

git commit -am 'Backport PR #896: Notify ChannelQueue that the response router thread is finishing'

4. Push to a named branch:

git push YOURFORK 1.x:auto-backport-of-pr-896-on-1.x

5. Create a PR against branch 1.x, I would have named this PR:

"Backport PR #896 on branch 1.x (Notify ChannelQueue that the response router thread is finishing)"

And apply the correct labels and milestones.

Congratulations — you did some good work! Hopefully your backport PR will be tested by the continuous integration and merged soon!

Remember to remove the Still Needs Manual Backport label once the PR gets merged.

If these instructions are inaccurate, feel free to suggest an improvement.