Add a prefix to the statsd key

We have loads of stats at the top leve of our statsd stats in graphite.
It makes looking for things that aren't created by backdrop really
hard.
Merge pull request #296 from alphagov/add-statsd-prefix

Add a prefix to the statsd key
Fix hostname check in replicate script

The hostname of the machine changed when it moved into the pp-puppet
repo from pp-development. It's now called `development-1` rather than
`vm`.
Merge pull request #303 from alphagov/fix-replicate-script

Fix hostname check in replicate script
Merge pull request #297 from alphagov/refactor-data-layer-step-1

Refactor data layer step 1
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