OAIRepository: oai_get_recid() for merged/deleted records
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 <2e218f779e12714c314fad8ebf6b3041daa21337@cern.ch>

inveniosoftware/invenio,tiborsimko/invenio,inveniosoftware/invenio,tiborsimko/invenio