[Feature Request] provide a way for users to cancel or close an existing Listener in Election api

**Is your feature request related to a problem? Please describe.**
For `Election` api, provide a way to close an observed `Listener`.
For now, we can use the `observe` method in `Election`:
```
void observe(ByteSequence electionName, Listener listener);
```
to add a listener, but it does not provide a way for users to cancel or close the listener.

**Describe the solution you'd like**
Provide a way for users to close or cancel the listener.