Mechanism for removing connection `onClose` callback

[viglucci]

We can register a callback with an RSocket using the onClose method. We do not currently have any way to deregister a previously registered callback.

Motivation

I'm sure there are other use cases, however, this would be useful when paired with the useEffect hook in React, which expect to be able to "undo" registrations and side-effects.

useEffect(() => {
  const id = rsocket.onClose((e) => {
    console.log(e);
  });
  return () => {
    rsocket.removeCloseListener(id);
  };
}, [rsocket]);

Desired solution

RSocket, likely through Closeable, could be extended to allow deregistration of callbacks. This might be best with having Closeable more closely resemble event emitter APIs.

// add a listener
const id = rsocket.onClose.addEventListener(() => {...});

// remove a listener
rsocket.onClose.removeEventListener(id);

Considered alternatives

Rsocket.onClose could implement/expose an observable that would natively support subscriptions. This could use existing types such as Cancellable and OnTerminalSubscriber.