Expose some sort of `horizon_stats` table

[dalanmiller]

It'd be cool to get realtime information from the Horizon server one is connected to similar to the rethinkdb db and it's associated tables. It might include:

• Current node process memory usage
• Number of connected clients
• Number of open changefeeds (nice for debugging)
• Current rate of queries per second
• Information about the RethinkDB instance it's currently connected to?

And many more! I'm not too sure how much sense it would be to write this information to a RethinkDB table constantly from the Horizon process, but maybe one could just query this RESTfully.

[deontologician]

I think we should send this over the websocket if we can

[segphault]

This is one of the changes that I'm working on in my adminui branch.

[deontologician]

This could also be a plugin