Support for canonical translations.

The TranslationAdaptor now has a fetch_all_by_Transcript() method with
which it's possible to get all translations for a transcript.

The fetch_by_Transcript() method now uses the new
canonical_translation_id field in the transcript table to get the
canonical transcript.

For species without alternative initiation (all Core speaces today), the
semantics and the results are the same as before.

Using the API from now on, without patch_56_57_e.sql applied, will
result in SQL errors.