Checking the state of the Node in NotifyLeave

I would like to check whether the service was stopped gracefully in `NotifyLeave`. The callback accepts a `Node` with a `State` field, but it is always equal to `StateAlive`.

Having looked at the code, I believe that the problem is that the `State` field is simultaneously declared in both `Node` and `nodeState` structs, while `Node` is nested in `nodeState`. Could someone explain if there are reasons to do this?

I see several solutions, but unfortunately they are not backwards compatible:
- add a getter and setter for `nodeState`, which will also change `Node.State`. At the same time, make the `State` field private.
- remove `State` field from `nodeState`
- remove the `State` field from `Node`, and add a separate parameter with the state to `NotifyLeave`

Start with NotifyLeave and the Node and nodeState definitions, then trace how State is populated before the callback. Confirm why the callback observes StateAlive and review the backwards-compatibility impact of the proposed API directions; done requires an agreed resolution to the state-reporting behavior.

Written by the indexing model from the issue text.