lib/weakhash, lib/model, cmd/syncthing: Decide if to use weakhash on startup (fixes #3938)

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3949

lib/util/utils.go

@@ -25,6 +25,17 @@ func SetDefaults(data interface{}) error {
 
 		v := tag.Get("default")
 		if len(v) > 0 {
+			if parser, ok := f.Interface().(interface {
+				ParseDefault(string) (interface{}, error)
+			}); ok {
+				val, err := parser.ParseDefault(v)
+				if err != nil {
+					panic(err)
+				}
+				f.Set(reflect.ValueOf(val))
+				continue
+			}
+
 			switch f.Interface().(type) {
 			case string:
 				f.SetString(v)