Transit: fix race in the key update api responses

[stevendpclark]

Description

• The key update API would release the lock a little too early after it persisted the update so the reference could be updated when it was preparing the response to the caller across updates and/or key rotations
• The storage updates were okay, just the response back to the caller of the update might see a mixture of different updates

TODO only if you're a HashiCorp employee

• Backport Labels: If this PR is in the ENT repo and needs to be backported, backport
  to N, N-1, and N-2, using the backport/ent/x.x.x+ent labels. If this PR is in the CE repo, you should only backport to N, using the backport/x.x.x label, not the enterprise labels.
  • If this fixes a critical security vulnerability or severity 1 bug, it will also need to be backported to the current LTS versions of Vault. To ensure this, use all available enterprise labels.
• ENT Breakage: If this PR either 1) removes a public function OR 2) changes the signature
  of a public function, even if that change is in a CE file, double check that
  applying the patch for this PR to the ENT repo and running tests doesn't
  break any tests. Sometimes ENT only tests rely on public functions in CE
  files.
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• RFC: If this change has an associated RFC, please link it in the description.
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  description. Also, make sure the changelog is in this PR, not in your ENT PR.

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CI Results:
All Go tests succeeded! ✅

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[kitography]

That makes sense to me 👍