RNAProductAdaptor: implement fetch_all_by_external_name()
RNAProductAdaptor: implement fetch_all_by_external_name()

Needs some extra code in DBEntryAdaptor to actually work.

muffato/ensembl,james-monkeyshines/ensembl,Ensembl/ensembl,muffato/ensembl,james-monkeyshines/ensembl,james-monkeyshines/ensembl,Ensembl/ensembl,Ensembl/ensembl,james-monkeyshines/ensembl,muffato/ensembl,muffato/ensembl,Ensembl/ensembl