Fix pending kernels again

[blink1073]

• Make the ready promise less timing-sensitive.
• Separate out a shutdown_ready promise.
• Always return concurrent future in synchronous manager and asyncio future in async manager.

[blink1073]

Server fix in jupyter-server/jupyter_server#1014

[Zsailer]

This is slick! Thanks, @blink1073!

[Zsailer]

As discussed, that's backport this to 7.x and release, since this fixes a bug in the pending kernels API.

[blink1073]

@meeseeksdev please backport to 7.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 7.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 c33e6b6cc3ebbed1e1a963dbe049599717f6a41b

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

git commit -am 'Backport PR #845: Fix pending kernels again'

4. Push to a named branch:

git push YOURFORK 7.x:auto-backport-of-pr-845-on-7.x

5. Create a PR against branch 7.x, I would have named this PR:

"Backport PR #845 on branch 7.x (Fix pending kernels again)"

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.