stats: add nesting around servers for easier parsing

[jbdalido]

Adding a simple nesting with key servers for easier parsing of the json delivered by /info on the api.

[shailesh33]

I am not exactly sure if this affects the dynomite-manager to collect stats. I will have to verify that.

[jbdalido]

For now the /info is mostly unusable for clean parsing, we are building an external prometheus exporter for a production use-case and it could be great to offer a cleaner output. Thanks a lot