v2.1.0 zero pinyin supports sync and async mode.

- bugfix on zero-input-pinyin: sometimes after GetCandidate dbus call,
  zero-input-fetch-size and zero-input-candidates is not updated. This
  was because the async function callback didn't update the metadata in
  some places.
- zero pinyin supports sync and async mode.
  to switch between them, run
  M-x zero-input-pinyin-enable-async
  M-x zero-input-pinyin-disable-async