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.

lib/sync/debug.go

@@ -11,7 +11,6 @@ import (
 	"strconv"
 	"time"
 
-	deadlock "github.com/sasha-s/go-deadlock"
 	"github.com/syncthing/syncthing/lib/logger"
 )
 
@@ -22,8 +21,7 @@ var (
 	// We make an exception in this package and have an actual "if debug { ...
 	// }" variable, as it may be rather performance critical and does
 	// nonstandard things (from a debug logging PoV).
-	debug       = logger.DefaultLogger.ShouldDebug("sync")
-	useDeadlock = false
+	debug = logger.DefaultLogger.ShouldDebug("sync")
 )
 
 func init() {
@@ -31,10 +29,4 @@ func init() {
 		threshold = time.Duration(n) * time.Millisecond
 	}
 	l.Debugf("Enabling lock logging at %v threshold", threshold)
-
-	if n, _ := strconv.Atoi(os.Getenv("STDEADLOCKTIMEOUT")); n > 0 {
-		deadlock.Opts.DeadlockTimeout = time.Duration(n) * time.Second
-		l.Debugf("Enabling lock deadlocking at %v", deadlock.Opts.DeadlockTimeout)
-		useDeadlock = true
-	}
 }