Proposed changes

As in #2904, control flow was simplified by replacing fragmented control flow and hashmaps with tokio tasks and regular async/await.

A test was added to confirm that requests are still processed concurrently.

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Improvement (general improvements like code refactoring that doesn't explicitly fix a bug or add any new functionality)
• Documentation Update (if none of the other choices apply)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Paste Link to the issue

Checklist

• I have read the CONTRIBUTING doc
• I have signed the CLA (in all commits with git commit -s)
• I ran cargo fmt as mentioned in CODING_GUIDELINES
• I used cargo clippy as mentioned in CODING_GUIDELINES
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

Further comments

Commits are kept small for ease of reviewing, once reviewed I'll merge them and add a description, unless it's not desired.

Fragmented uploads and downloads are only left in FileCacheActor and LogManagerActor, I will do them in next PRs.