Refactor mongo db access and cleanup

- Rename from storage to mongo to be explicit about the fact we're dealing
  with a pymongo object here.
- Add a common clean_mongo method to do the cleanup
- Move mongo methods into the base client as they're part of the commong
  interface.
Remove Database from write and admin apps

It is no longer needed, MongoStorageEngine is used instead.
Set '_updated_at' when a record is saved

The '_updated_at' should when the record was last updated. This was
missed in the earlier save work.
Merge pull request #300 from alphagov/refactor-data-layer-step-4

Refactor data layer step 4