Status refactor 1: Refactor membership to use localMember as the source of truth

[mennopruijssers]

Use a in-memory object to represent once local state. This makes the code path that updates the local state more robust. Instead of changing the state of the current node by updating it using updates, we just change the localMember. Whenever a gossip comes in changing the current state, we can always "counter-gossip" with the current state.

Note: this is the first PR of a serie of refactors:

• [x] Use a local member object to maintain local state: #291 (this PR)
• [x] Use a common method for status updates: #292
• [x] Remove LeaveUpdate-object: #293
• [x] Remove "unsupported" state update methods: #294
• [x] Add a populate{Subject,Source} to the Change Object: #296
• [x] Add a _reincarnate-method to centralize reincarnation logic: #291 (this PR)

[thanodnl]

LGTM

[motiejus]

Besides the incarnation number change (non-blocking, just let me know what you think), LGTM.

[mennopruijssers]

As discussed offline with @thanodnl I renamed shouldProcessChange to shouldProcessGossip. He's doing the same in ringpop-go so we have them in sync. (Because I already merged in the other PR's it's easier to just check: https://github.com/uber/ringpop-node/pull/291/commits/17ae4060b67d124812ac98e7aae9b281a88b86c6)