SWIM: Remove ack incarnation, use gossip about self instead?

[ktoso]

Proposal by @avolokhov

---

Remove incarnation from ack payload and put information about pinged instance into response payload.
This will deduplicate logic of membership dissemination code and condense it in gossip payload processing.

Motivation:

Current SWIM protocol has two ways to notify about group membership: membership about everyone except pinged instance is disseminated in gossip payload, and membership about self is stored in a dedicated incarnation field. This forces us to have essentially the same membership processing code in two places. This PR removes this duplication and makes gossip payload the only way to pass group membership information.

Modifications:

• SWIM Ack ping response no longer contains incarnation field.
• SWIM gossip payload will always carry information about pinged instance.
• SWIMShell will only modify group membership information based on gossip payload

---

Need to consider the deeper impact of such change.