lib/model: Use a single lock (phase two: cleanup) (#9276)

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.

go.mod

@@ -40,7 +40,6 @@ require (
 	github.com/prometheus/procfs v0.12.0 // indirect
 	github.com/quic-go/quic-go v0.40.0
 	github.com/rcrowley/go-metrics v0.0.0-20201227073835-cf1acfcdf475
-	github.com/sasha-s/go-deadlock v0.3.1
 	github.com/shirou/gopsutil/v3 v3.23.11
 	github.com/syncthing/notify v0.0.0-20210616190510-c6b7342338d2
 	github.com/syndtr/goleveldb v1.0.1-0.20220721030215-126854af5e6d
@@ -68,7 +67,6 @@ require (
 	github.com/matttproud/golang_protobuf_extensions/v2 v2.0.0 // indirect
 	github.com/onsi/ginkgo/v2 v2.13.2 // indirect
 	github.com/oschwald/maxminddb-golang v1.12.0 // indirect
-	github.com/petermattis/goid v0.0.0-20231126143041-f558c26febf5 // indirect
 	github.com/power-devops/perfstat v0.0.0-20221212215047-62379fc7944b // indirect
 	github.com/prometheus/client_model v0.5.0 // indirect
 	github.com/quic-go/qtls-go1-20 v0.4.1 // indirect