Postgres benchmark hits read

Generalise the simpler rake task out into a couple of benchmark classes. We now have a task that can be used like this:

```sh
  rake benchmark:hits:view[hmrc,summary,all-time,10]
```

All of the params are optional; will default to Ofsted, all hits, all time, 5 runs.
Could you outdent `private`, as the [style guide suggests](https://github.com/alphagov/styleguides/blob/master/ruby.md#classes)?