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>