OAIRepository: oai_get_recid() for merged/deleted records

• Reimplements oai_get_recid() to be aware of merged or deleted records (preferring to return recid for existing and non deleted records when more than one record matches a given recid). (closes #1429)

Signed-off-by: Samuele Kaplun <samuele.kaplun@cern.ch>