Add readiness API to sensu-go API

[c-kruse]

New serve-wait-time flags expose a method to delay servicing api and agent traffic. This information needs to be communicated back to consumers. Today sensu-go's apis have no concept of readiness aside from whether or not they have bound and are accepting connections and the /health endpoint is a poor location for this information.

What

Add a new /ready api endpoint to the sensu-go APIs (the sensu-go API as well as the agent listener). /ready will return a HTTP 200 when the API is ready to serve traffic, and will otherwise fail (503).