lib/model: Incremental block stats usage reporting

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/4498
LGTM: calmh

cmd/syncthing/mocked_model_test.go

@@ -115,6 +115,6 @@ func (m *mockedModel) State(folder string) (string, time.Time, error) {
 	return "", time.Time{}, nil
 }
 
-func (m *mockedModel) UsageReportingStats(version int) map[string]interface{} {
+func (m *mockedModel) UsageReportingStats(version int, preview bool) map[string]interface{} {
 	return nil
 }