Performance improvement by *not* splitting OR into multiple queries

(MySQL v5 knows how to do index merges) in _type_by_external_id():

  # decrease in time by 35%
  $dbea->list_transcript_ids_by_extids('BRCA2');

  # decrease in time by 28%
  $dbea->list_translation_ids_by_extids('GO:0004835');

  # decrease in time by 24%
  $dbea->list_gene_ids_by_extids('ARSE');


Also some formatting.