Reduce the number of statsd keys we use.

When we use a custom statsd key for every search term, tag filter or
SNAC code, each one consumes disk space on our Graphite box. This gets
very large very quickly. Since we aren't making extensive use of
Graphite to monitor the content API, we can pare this down for now.
Merge pull request #111 from alphagov/reduce-statsd-use

Reduce the number of statsd keys we use.