Improve proxy vhost sensu checks

- Measure on the total number of matching requests over the previous 1
  minute. `keepLastValue` means that the line doesn't drop to zero until
  all non-zero records have fallen out of the window. It is also a bit
  harder to understand.
- Set graphite sensu checks to be empty if `$sensu_check` is switched
  off. Apparently this is the only way sensu checks can be disabled
  right now.
- Add more lenient 4xx limits for monitoring apps