Add gossip protocol for cluster membership.

[mikecdavis]

Summary

• Adds hashicorp/memberlist dependency
• Creates "broadcasts" for config init/update and override set/remove
• Defines node "state" by loaded datafiles and current set of overrides
• Uses vendored MapExperimentOverridesStore from the go-sdk
• Adds top level admin index route
• Adds start_node script for creating a local clustered environment

Agent nodes within the same cluster should maintain some degree of consistency between them. Especially in the context of data file existence and revisions the

TODO

• Update the MapExperimentOverridesStore in the go-sdk for direct access to the underlying map
• Add unit tests