Add StakesCache struct to abstract away locking (backport #21738)

[mergify]

This is an automatic backport of pull request #21738 done by Mergify.
Cherry-pick of fd175c1 has failed:

On branch mergify/bp/v1.8/pr-21738
Your branch is up to date with 'origin/v1.8'.

You are currently cherry-picking commit fd175c1ea.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   runtime/src/serde_snapshot/future.rs

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   runtime/src/bank.rs
	both modified:   runtime/src/serde_snapshot/tests.rs
	both modified:   runtime/src/stakes.rs

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/github/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

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[mergify]

automerge label removed due to a CI failure

[jstarry]

@Mergifyio refresh

[mergify]

refresh

✅ Pull request refreshed

[jstarry]

@Mergifyio rebase

[mergify]

rebase

☑️ Nothing to do

Details

• -closed [:pushpin: rebase requirement]
• #commits-behind>0 [:pushpin: rebase requirement]