This system tracks an abstract "api version" from nodes based on
their usage, allowing us to have latching behavior where if a node
ever uses a new api, it can be blocked from using the old api.
This is better than using self-reported semver version information
because the node cannot lie, there's no confusion about what semver
version implies which features, no questions about dev and ci
environments, and no dependencies between reporting the version
and using the new api.
Change-Id: Ifeced5c9ae8e0a16102d79635e176a7d3bdd8ed4
