[performance] simpler throttling logic

[NyaaaWhatsUpDoc]

Description

Forgot I had these changes waiting around in the mountain that is my git stash >:D

Reduces the complexity of the throttling middleware to use a singular channel queue for the permitted executing requests logic, and simplifies the backlog to only use an atomically updated integer (we don't need anymore than this).

Checklist

• I/we have read the GoToSocial contribution guidelines.
• I/we have discussed the proposed changes already, either in an issue on the repository, or in the Matrix chat.
• I/we have performed a self-review of added code.
• I/we have written code that is legible and maintainable by others.
• I/we have commented the added code, particularly in hard-to-understand areas.
• I/we have made any necessary changes to documentation.
• I/we have added tests that cover new code.
• I/we have run tests and they pass locally with the changes.
• I/we have run go fmt ./... and golangci-lint run.