Return all adaptors in the registry in get_all_DBAdaptors if no args supplied

Previously, when no args (either species or group) supplied, the implementation
at get_all_DBAdaptors requires looping through '_DBA' array and returns an array
of each DBA's reference. It takes long time if there are large number of DBAs
(e.g. >0.5s for ~88k DBAs).

This change returns the reference to the '_DBA' array itself directly in such cases.