Issue with reference mapping for ENA

[blakesweeney]

@AntonPetrov found that some sequences are missing a mapping from the accession to the reference. He found the sequences with:

select *
from xref t1, rnc_reference_map t2
 where
    t2.ACCESSION (+) = t1.ac and
    t2.accession is null and
   deleted = 'N';
 -- 17067

I looked into this a bit and they are all ENA sequences. The sequences tend to be fly sequences but not always. It doesn't appear that there is a single publication that is missing. I tried to do some more looking but got myself confused. This is a bit of a longer term sequence to figure out exactly what the issue is and how to fix it.