add RGD as official source for rat
rat is now a merged species
RGD have direct mappings to Ensembl stable ids, use as direct xrefs
ENSCORESW-970: CCDS xrefs are associated to genes and transcripts
we want only the transcript xrefs
xref and object_xref entries are not always unique
allow for insert ignore
check if entry already exists and use existing xref_id/object_xref_id for further insertions
attempting to transfer dependent xrefs correctly when assessing xref with best priority
allow for duplicated object_xref entries if master_xref is different