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. (PR #2525)

• 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>
Reviewed-by: Tibor Simko <tibor.simko@cern.ch>