extend namespaces api response to include current limits/quota

[rabbah]

there isn't a way for a subject to check how much of their quota they've already consumed - we can potentially provide this information by extending the GET /namespaces api to include the current counts and limits for the subject.

[markusthoemmes]

We need to be wary here (and/or rethink our strategies) that in an HA setup, these values are subject to eventual consistency and as such might not be helpful for the user (depending on the use-case she is trying to implement).