Retry messages on db uniqueness validation errors.

These will occur if two consumers try to create the same item
simultaneuosly (due to multiple messages for the same content_id coming
through very close together).  Retrying the message will result in it
becoming an update next time it's processed.
Merge pull request #19 from alphagov/retry_on_db_uniqueness_violation

Retry messages on db uniqueness validation errors.