Summary:
This adds the options to `eden stats` for collecting only fast stats and printing in JSON.
`eden stats` can be slow especially due to collecting fb303 counters and private bytes. An example use case of this new lightweight endpoint is that Buck can poll it to display Eden related info in its cli (see [post](https://fb.workplace.com/groups/132499338763090/permalink/210396380973385/) for context).
Reviewed By: xavierd
Differential Revision: D29687041
fbshipit-source-id: a663e71231527c5dfb822acbf238af0ac6ce4a00