Add stats response to the management API

[hjr3]

We are now tracking success and response at the pool level. We need to wire up the /stats response in the management API.

[maniankara]

@hjr3 Some questions about this:

1. Seems there is no tests for the api part?
2. There is so much copy-paste of this? No framework/tool for this?

Link { rel: "add".to_string(), href: "/servers".to_string(), method: Some("POST".to_string()), },

Despite of all these, I am issuing a pull request :)