service discovery: add RPC endpoints and FSM logic

[jrasell]

This PR adds the RPC logic for service discovery. This includes a new endpoint as well as updates to the alloc and job endpoints to provide easy lookup of registrations associated to these objects.

The new ServiceRegistration endpoint is static/dynamic to ensure the Upsert RPC, which is only callable via client agent, includes TLS verification. It also includes functionality for wildcard listing as is common across all list endpoint within Nomad.

The ipaddr package mirrors Consul and ensure no service is registered which has an any IP address and therefore is not logically routable outside its local instance.

Apologies for the size, although I believe at least 60% of the additions are tests. The PR is split into commits which group the additions into logical chunks and hope to make it easier to review.

closes https://github.com/hashicorp/team-nomad/issues/259

[tgross]

LGTM. I've left one question/comment about TLS cert checking that we should look at, but other than that this is really solid.

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