OAIRepository: lower priority to updating uploads

* Attributes lower priority to updating uploads (generated
  by oai_repository_updater). Also, assigns a specific
  -N argument so that these bibuplods can run concurrently
  to other.

* Checks whether other uploads from previous execution of
  oai_repository_updater need still to be elaborated by
  the queue. In this case skip the execution of
  oai_repository_update.

Signed-off-by: Samuele Kaplun <samuele.kaplun@cern.ch>