Add extended apikey stats

This adds new stat counters to track if we responded with the best possible position estimate given the incoming request data. We only want to enable this one some of the api keys, like those used for actual Firefox OS devices.

The changes are done in two parts, one commit to clean up test failures based on us doing wifi data validation earlier in the chain. The other implementing the actual additional stats. And I also snuck in the missing metric docs for the new Redis update queues and removed some long repetitive lists from the same docs.

@jaredkerim f?
This changes from checking whether the 'signal' key is present in the dict, and also that it is an integer.  Is that safe?