Refactor agent startup logic and experimental os.fork support

[Superskyyy]

This PR is made of two tightly coupled parts:

1. Total rewrite of agent startup logic from module functions -> singleton class. (some other logic was changed in meter to fix wrong forking behavior)
2. Provide experimental support for os.fork(), exposed as an option.
3. A demo directory to provide easier access to oap/kafka/demoservices (for contributors).

Minor changes:

1. Docs: fixed some missed ones over time.
2. Fixed a redis bug.

• If this pull request closes/resolves/fixes an existing issue, replace the issue url. Closes: [Python] fork() support for grpc protocol skywalking#7319
• Update the CHANGELOG.md.