Replace utcnow().timestamp() with time.time() in auth_store

[bdraco]

Proposed change

utcnow().timestamp() is a slower way to get time.time()

Type of change

• [ ] Dependency upgrade
• [ ] Bugfix (non-breaking change which fixes an issue)
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• [ ] Breaking change (fix/feature causing existing functionality to break)
• [x] Code quality improvements to existing code or addition of tests

Additional information

• This PR fixes or closes issue: fixes #
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