lib/connections: Allow on the fly changes to rate limits (fixes #3846)

Also replaces github.com/juju/ratelimit with golang.org/x/time/rate as
the latter supports changing the rate on the fly.

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3862

cmd/syncthing/main.go

@@ -637,9 +637,6 @@ func syncthingMain(runtimeOptions RuntimeOptions) {
 		},
 	}
 
-	// If the read or write rate should be limited, set up a rate limiter for it.
-	// This will be used on connections created in the connect and listen routines.
-
 	opts := cfg.Options()
 
 	if !opts.SymlinksEnabled {