Bug 3835: Design and implement OAI-PMH compliant harvest subsystem
1. Refactor the 'eml' metadataPrefix into a distinct metadataPrefix value (and corresponding crosswalk class) for each EML version:
eml-2.0.0, eml-2.0.1, eml-2.1.0. (In recent VTC discussion, we decided not to add support for the older 'beta' versions of EML 2.0.0.)
2. Implement logic for filtering results returned by the ListIdentifiers and ListRecords verbs based on selection criteria
such as 'from', 'until', and 'metadataPrefix'.