clean up and refactor active record container backend

[doits]

fixes dirty tracking and creating empty translation hashes on read

fixes #540

[shioyama]

Thanks for this! ❤️ Will review this week.

[doits]

I rebased everything and updated as requested:

• Always assume model[column_name] is a hash
• No compact anymore
• One spec about removing empty locale hashes when last attribute is deleted

One thing to keep in mind is that empty hashes might already exist in saved models if the container backend was used before this PR and it will not "self heal" (like it did with compact). Not sure what to do about it though, or if there should be done anything about it.

Edit: I did it as extra commits so you can follow it. I can squash all commits if everything is good at the end.

[doits]

Hey @shioyama, hope you're good! Any chance to review this PR again? I'd like to get this merged and off my list :-) If there are some changes needed I'll update the PR, just say so.

(Edit: Just rebased it on master)

[shioyama]

@doits Thanks so much, this is a really great PR! ❤️

One thing to keep in mind is that empty hashes might already exist in saved models if the container backend was used before this PR and it will not "self heal" (like it did with compact). Not sure what to do about it though, or if there should be done anything about it.

I think that's ok, if it hasn't been a problem so far then presumably it won't suddenly start being one (for those using the container backend).