Cleanup after #9275.
This renames `fmut` -> `mut`, removes the deadlock detector and
associated plumbing, renames some things from `...PRLocked` to
`...RLocked` and similar, and updates comments.
Apart from the removal of the deadlock detection machinery, no
functional code changes... i.e. almost 100% diff noise, have fun
reviewing.
Other routines use atomics, hence even if we are under a lock, we should
too.
We might atomically store with
Not sure how it happens, but it's between lines
GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3974
