Move benchmark:import_hits to benchmark:hits:import

* More consistent with what we've done elsewhere
* Allows us to use the same benchmark:hits:all trick later
* Move the actual import to a Benchmark module. We can't have
  multiple top-level same-named constants in .rake files, as rake
  loads all files, not just the one we care about/are invoking.
Add a benchmark rake task for all hits, all time

* Default to Ofsted
* Doesn't really simulate the view portion (we'd need a full-on
  Poltergeist for that. Not sure how hard (or not) that is)
* We only care about Postgres timings at this stage
Generalise the :all_hits_all_time task to just :view

* Can then accept params, e.g.
   `rake benchmark:hits:view[hmrc,summary,last-30-days,10]` or
   `rake benchmark:hits:view[hmrc,redirects,all-time,10]`
   `rake benchmark:hits:view[ofsted,hits,all-time,10]`
* Given no params, defaults to Ofsted, all hits, all time
Merge pull request #412 from alphagov/postgres-benchmark-hits-read

Postgres benchmark hits read