Rename `Async::LimitedBarrier` to `Async::Waiter` based on feedback.

[ioquatix]

Based on feedback, renaming. It was not released yet, so this is acceptable.

Types of Changes

• Maintenance.

Contribution

• [x] I added tests for my changes.
• [x] I tested my changes locally.
• [x] I agree to the Developer's Certificate of Origin 1.1.