chore(deps): switch to the stdlib atomic package in snapshot code

[Stebalien]

Related Issues

We're using uber's atomic instead of the builtin one in our snapshot logic. We probably used uber's by mistake. It comes into our dependency tree via go-ipld-blockstore and will leave via filecoin-project/filecoin-ffi#497.

Proposed Changes

Switch to the builtin atomic package.

Checklist

Before you mark the PR ready for review, please make sure that:

• Commits have a clear commit message.
• PR title conforms with contribution conventions
• Update CHANGELOG.md or signal that this change does not need it per contribution conventions
• New features have usage guidelines and / or documentation updates in
  • Lotus Documentation
  • Discussion Tutorials
• Tests exist for new functionality or change in behavior
• CI is green