Don't return db heavy stats from the status API call by default

[einsteinx2]

The status API call needs to be separated into "basic" and "extended" info. Where basic is returned by default, and extended only included when specifically asked for. That way we can keep all kinds of info in the extended response, but keep the status call quick for frequent use by clients.

Also if possible, we should cache the values once they've been read until the db is modified again.

[mdlayher]

Done, see #71.

No caching yet, but this will alleviate the database strain. We could probably just keep an object in memory which contains these metrics, and updates them whenever the last query ID changes.

[mdlayher]

And now we have caching! That was easy.

[justindhill]

Seems legit.