refactor: use context.AfterFunc around VU management

[mstoykov]

What?

Use context.AfterFunc

Why?

This cuts down on the amount of goroutines needed to be up for each VU.

Checklist

• [ ] I have performed a self-review of my code.
• [ ] I have added tests for my changes.
• [ ] I have run linter locally (make lint) and all checks pass.
• [ ] I have run tests locally (make tests) and all tests pass.
• [ ] I have commented on my code, particularly in hard-to-understand areas.

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