Make the cache simpler, seriously

[andersfylling]

Description

I've come to realize I don't have the time to maintain some complex cache implementation, so I'm simplifying it to make it easier to read / maintain. Performance is not negatively affected. This time I'm adding test cases for different scenarios, even deadlock testing, in hope that we can get some more stability.

This should also help contributors.

fixes https://github.com/andersfylling/disgord/issues/396

Type of change

• [x] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• [x] I ran go generate
• [x] I have performed a self-review of my own code (remember to run go fmt ./...)
• [x] I have commented my code, particularly in hard-to-understand areas
• [ ] I have made corresponding changes to the documentation
• [x] I have added tests that prove my fix is effective or that my feature works
• [x] New and existing unit tests pass locally with my changes
• [ ] Any dependent changes have been merged and published in downstream modules
• [ ] Added benchmarks if this is a performant required component (potential bottlenecks)