Replace async Event with bool

[Olegt0rr]

Description

@Lancetnik asked to replace the awaiting of asyncio.Event in favor of asyncio.sleep in a loop, so that the implementation would be similar to uvicorn:
https://github.com/encode/uvicorn/blob/079f07a06457717b6f8c4b23bab7674d5f0568c1/uvicorn/server.py#L222-L229

The solution with asyncio.Event in the __init__ did not allow amateurs to use global object storage to use the library correctly, cause event instance became attached to another event loop.

I proposed an option to ensure compatibility with the use of asyncio.Event with lazy creation, but @Lancetnik asked for asyncio.sleep only. So I copy solution from unicorn.

Fixes #1837

Type of change

Please delete options that are not relevant.

• Documentation (typos, code examples, or any documentation updates)
• Bug fix (a non-breaking change that resolves an issue)
• New feature (a non-breaking change that adds functionality)
• Breaking change (a fix or feature that would disrupt existing functionality)
• This change requires a documentation update

Checklist

• My code adheres to the style guidelines of this project (scripts/lint.sh shows no errors)
• I have conducted a self-review of my own code
• I have made the necessary changes to the documentation
• My changes do not generate any new warnings
• I have added tests to validate the effectiveness of my fix or the functionality of my new feature
• Both new and existing unit tests pass successfully on my local environment by running scripts/test-cov.sh
• I have ensured that static analysis tests are passing by running scripts/static-analysis.sh
• I have included code examples to illustrate the modifications