readded identity_xref column fetching/storing that was lost during merge with branch-new-seqstore
readded identity_xref column fetching/storing that was lost during merge with branch-new-seqstore

willmclaren/ensembl,muffato/ensembl,james-monkeyshines/ensembl,james-monkeyshines/ensembl,Ensembl/ensembl,Ensembl/ensembl,at7/ensembl,at7/ensembl,mjg17/ensembl,mjg17/ensembl,muffato/ensembl,at7/ensembl,willmclaren/ensembl,at7/ensembl,mjg17/ensembl,Ensembl/ensembl,muffato/ensembl,Ensembl/ensembl,danstaines/ensembl,mjg17/ensembl,muffato/ensembl,danstaines/ensembl,james-monkeyshines/ensembl,james-monkeyshines/ensembl,danstaines/ensembl