atelet: fast-forward a Checkpoint already at its destination

[Troy Chiu (troychiu)]

Part 5 of a 5-part split. Based on main rather than the branch below it, so the diff also carries the earlier branches' commits; the commit to review here is the last one, atelet: fast-forward a Checkpoint already at its destination.

Fixes #<issue_number_goes_here>

It's a good idea to open an issue first for discussion.

• Tests pass
• Appropriate changes to documentation are included in the PR

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The stack (each PR is based on main, so each diff contains the ones above it):

1. ategcs: recognize every S3 "not there" as object absence #1135 — ategcs: recognize every S3 "not there" as object absence
2. microvm: find a restored actor's CH api-socket without ateom's record #1136 — microvm: find a restored actor's CH api-socket without ateom's record
3. atelet: serialize node-local operations per actor #1137 — atelet: serialize node-local operations per actor
4. ateom: replay a completed checkpoint instead of re-running it #1138 — ateom: replay a completed checkpoint instead of re-running it (+ ateomnet netns cleanup guard)
5. atelet: fast-forward a Checkpoint already at its destination #1139 — atelet: fast-forward a Checkpoint already at its destination

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