[bugfix] fix possible mutex lockup during streaming code

[NyaaaWhatsUpDoc]

Description

This fixes an issue I have noticed a few times on my own instance. It rewrites Stream{} to use much less mutex locking, and updates related code calling it.

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 not leveraged AI to create the proposed changes.
• 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.

[daenney]

The storing a context on the Stream feels wrong. It's something the context package warns against doing and the Go team wrote a whole blog post about. We might have a good reason for it, but I don't know what the motivation for it is and why it has to be done this way.

[NyaaaWhatsUpDoc]

The storing a context on the Stream feels wrong. It's something the context package warns against doing and the Go team wrote a whole blog post about. We might have a good reason for it, but I don't know what the motivation for it is and why it has to be done this way.

ah i wasn't aware of this. will have a think about rearchitecting it, and mark it draft for now. specifically it's the 'Done()' channel part of the context that's a useful piece of logic for tracking only-once pieces of behaviour like closing and being able to select against it, but i suppose i can just use a channel separately for that. (that was actually what i did in previous iterations of this code but i ended up going with context.Context because reasons i can't remember)

have rearchitected with this now in mind :)

[NyaaaWhatsUpDoc]

just wanna make one last change, changing to draft until then even though all tests are passing