Use ActiveRecord::QueryCache as middleware

Avoids repeated queries to the database for queries whose strings are identical
in the context of the current request.

vagrant@development:/var/govuk/bouncer$ bundle exec rake benchmark:all
                user     system      total        real
  no host:  0.940000   0.070000   1.010000 (  1.152042)
  410:      0.660000   0.010000   0.670000 (  0.777661)
  301:      0.660000   0.010000   0.670000 (  0.767946)
  404:      0.650000   0.010000   0.660000 (  0.768585)