Adds network tomography features to Consul.

[slackpad]

This should complete network tomography on the Consul side. There was a pretty major rebase after the memdb changes, so the final commit (ecd3a1d) has the major changes associated with that. That change is worth a quick review, everything else has been reviewed previously.

Here are the biggest changes made during the rebase:

1. While in here I realized that the state store would accept coordinates for nodes that weren't in the catalog. Since deleting coordinates is tied to deleting nodes, and we don't really control if agents send coordinates before they have successfully been registered in the catalog, this seemed like a potential for inconsistency. I made the state store just skip coordinate updates for nodes not in the catalog, which required some changes to unit tests.
2. Got rid of the single coordinate get function since it was only used in unit tests.
3. Beefed up unit tests in some places.
4. Cleaned up the coordinate types in the structs package.

/cc @armon and/or @ryanuber

[ryanuber]

LGTM! 🚢