Implement last updated feature of mongo engine

Implement MongoStorageEngine.get_last_updated. Given a data-set name
return the most recent _updated_at timestamp. This is required to be
able to determine if a data-set has been updated recently enough to be
considered up to date.
Integrate last updated feature of mongo engine

This introduces a new data-set class called NewDataSet. This is a
temporary class for the duration of the refactor that, once complete can
be renamed to DataSet. The external interface of is_recent_enough stays
exactly the same, but internally it defers to the storage engine for to get
the last update time.

This also integrates is_recent_enough with the read API detailed status view
that makes use of it.
Implement empty data set feature of mongo engine

Implement MongoStorageEngine.empty method to allow a data set to be
emptied.
Integrate empty data set feature with write API
Merge pull request #298 from alphagov/refactor-data-layer-step-2

Refactor data layer step 2