🔨 Death to raw mutex and lock usage

[zach2good]

I affirm:

• I understand that if I do not agree to the following points by completing the checkboxes my issue will be ignored.
• I have read and understood the Contributing Guide and the Code of Conduct.
• I have searched existing issues to see if the issue has already been opened, and I have checked the commit log to see if the issue has been resolved since my server was last updated.

Describe the feature

We have mutex_guarded and shared_guarded wrappers which do a much better job than managing this sort of thing by hand. We should be using them instead.

[zach2good]

This is the last place outside of common utils that uses raw locks/mutexes, so I'd consider this done: #6357