peers: Use peer cache to store information for RPC routing.

[jrasell]

Note: new improvement, do not merge until after 1.11 GA.

Nomad 1.11 introduced a new peer cache to store information about Serf peers in order to perform server version checking. This sub-system provides a good place to store all cached Serf peer information which is also stored in the server object.

This change therefore moves the peer and local peer information from the server object into the peer cache backend. It means we have a single place to query this information as well as a single place to reflect Serf membership updates.

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Changes to Security Controls

Are there any changes to security controls (access controls, encryption, logging) in this pull request? If so, explain.

[tgross]

Just a heads up that you might want to look at the ENT code base for consumers of these fields, in particular the Autopilot ENT features.