This isn't needed/used yet, but it's a good hook to get in there so we
can add more atomic operations in the future. The Go API hides this detail
so that feels like a KV-specific API. The implications on the REST API are
pretty minimal.
This is necessary as consul-api's tests require a real consul instance
to be running. We can't directly import an agent to fire up an instance,
due to the way this would create an import cycle. These tests instead
will start a consul instance using the binary in $PATH (if it exists).
