In fetch_all_by_Slice_and_external_dbname_link(): Allow for non-unique

db_name in external_db. Clean up a few bugs while I'm at it.

Bugs corrected:

  A list (rather than a list reference) was returned if the
  external_db_id for the given db_name could not be found.

  The list returned from list_gene_ids_by_external_db_id() (in
  DBEntryAdaptor) was used as a hash.