Switching DoiCreationRequestJob to use header_id

Given that there is a database constraint on the DoiCreationRequest,
the repository/job interaction is easier to handle if I am passing the
header_id instead of the doi_creation_request_id.

I am also able to remove some object persistence.
Merge pull request #25 from ndlib/switching-doi-creation-request-to-header-id

Switching DoiCreationRequestJob to use header_id