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

[exceptionplayer]

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.

[lburgazzoli]

@medusar any time to contribute a pr ?

[exceptionplayer]

@medusar any time to contribute a pr ?

ok, I will do that when I am on vacation 3 weeks later