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
Merge pull request #298 from alphagov/refactor-data-layer-step-2

Refactor data layer step 2
Implement save method on MongoStorageEngine
Integrate storage engine save with write API

Integrate the new MongoStorageEngine save method with the write API and
admin app. The integration with the admin app is not right, but fixing
that at the same time is a bit too much.

This required a bit of refactoring.
1. Create a store method with the same interface as parse_and_store on
   NewDataSet
2. Refactor pre-processing steps to be a series of functions applied to
   the records and refactor tests
3. Add some tests around those steps being called
4. Integrate NewDataSet with the write API

This commit does not include removing the legacy saving code and tests.
Removes redundant saving code

Removes code related to the previous method of saving. See previous commits
for context.

- Remove saving from DataSet, it is being done in NewDataSet
- Remove Record, it is no longer necesarry, dicts are used instead.
- Remove test code associated with the dead code.
Add delete_dataset method to mongo storage engine

I missed this one in earlier commits and only realised when I tried to
remove the Database class from the write API.
Integrate dataset deletion against mongo storage