Postgres COPY ... FROM STDIN

* Avoid superuser
* Be faster
* That's it

See 77ec9cb commit message for timings.
Well, we've a list of four actions, to be executed in order, and I just wanted the lines together for readability's sake. The options I tried are:
1.
```ruby
  ActiveRecord::Base.connection.execute(TRUNCATE)
  copy_to_hits_staging(filename)
  ActiveRecord::Base.connection.execute(INSERT_FROM_STAGING)
  ActiveRecord::Base.connection.execute(UPDATE_FROM_STAGING)
```

or

2.
```ruby
  ActiveRecord::Base.connection.execute(TRUNCATE)
  copy_to_hits_staging(filename)
  ActiveRecord::Base.connection.execute(INSERT_FROM_STAGING)
  ActiveRecord::Base.connection.execute(UPDATE_FROM_STAGING)
```

or 

3.
```ruby
          [
            TRUNCATE,
            :copy_to_hits_staging,
            INSERT_FROM_STAGING,
            UPDATE_FROM_STAGING
          ].each do |statement|
            if statement == :copy_to_hits_staging
              copy_to_hits_staging(filename)
            else
              ActiveRecord::Base.connection.execute(statement)
            end
          end
```

or 

4.
```ruby
```