rename XS into RS, xs into rs, and excess into toubkal
js/dropbox:
- fix transaction reuse, was due to event fired on old _add_value() listeners that should only be called once
- add transaction number to traces to help debug transaction used
- rename _wait_for_configuration() into _wait_for_dropbox()
- rename _configuration_waiters into _dropbox_waiters
- remove_url(): emit nothing on no value
- _remove_value():
  - add ToDo for antistate management
  - use cache[ file_path ] to register event when it's a function
  - use remove_url() to emit nothing
- more comments