Added new optional argument $external_db_name to

fetch_all_by_external_name() in GeneAdaptor.pm, TranscriptAdaptor.pm,
and in TranslationAdaptor.pm.

This will limit the returned genes, transcripts, or translations, to
those that originates from the named external database.

This also means adding the optional argument to
list_gene_ids_by_extids(), list_transcript_ids_by_extids(),
list_translation_ids_by_extids(), and to _type_by_external_id() in
DBEntryAdaptor.pm.

The matching is done on 'db_name' in the 'external_db' table with LIKE
and an appended '%' to the given $external_db_name.