Filtering `bbl_translated_meta_fields` doesn't stop `meta_keys` from syncing

[simonwheatley]

1. Specify some fields for translation, as per https://github.com/Automattic/babble/blob/master/translation-fields.php
2. Create a post and trigger translations
3. Complete the translations
4. Update the original post

The translated fields will be overwritten by the values specified in the post in the canonical language.

Two possible solutions:

1. The developer should both filter bbl_translated_meta_fields to specify the translation config for each field AND filter bbl_sync_meta_key to stop those keys being filtered
2. Babble should stop a meta_key which has a translation config (e.g. in bbl_translated_meta_fields) from syncing

[simonwheatley]

See also #257