oaiharvester: record splitting improvement

• Improves the splitting of records from OAI-PMH requests to also handle the case of MARCXML from other Invenio instances.

• Defers the splitting logic to a XML parser and moves some utility functions to a new utils file in the new module structure.

• Adds tests for these utility functions.

Signed-off-by: Jan Aage Lavik <jan.age.lavik@cern.ch>