Check bucket health separate to _status endpoint

- `_status` just checks db health, as per
- `_status/buckets` checks bucket health
- Should allow us some flexibility with checks, and means varnish can deal with _status without kludging

Kanye get behind this?
![](https://24.media.tumblr.com/984a710733536aebb49cf6b2aa039c8c/tumblr_mwuzvqMywj1qccftlo1_500.jpg)