[WIP] Add migration helper methods [skip ci]

[fatkodima]

Related #95

Currently, I'm not asking for a thorough review, just POC validation.

This adds (documentation and usage examples provided inline with method definitions):

1. add_foreign_key_concurrently. If we can update activerecord dependency to >= 5.2, then this won't be needed as add_foreign_key ..., validate: false can be used directly
2. update_column_in_batches
3. add_column_with_default
4. rename_column_concurrently, undo_rename_column_concurrently, cleanup_after_rename_column_concurrently, undo_cleanup_after_rename_column_concurrently - for working with renaming columns
5. change_column_type_concurrently, cleanup_after_change_column_type_concurrently

While writing this, I have figured it out, that it will also be useful to add something like rename_table_concurrently. If you have other ideas of what can be useful to add, please share.

Gitlab also has some code for background migrations for extra large tables. I do not considered them here to make this PR more digestible. Those will be added separately, if needed.

Todo:

• support mysql
• add tests
• update documentation

[ankane]

Hey @fatkodima, thanks for another PR! Overall, I think migration helpers makes sense. A few notes:

1. Let's add them one at a time (or in logical groups) so we can discuss them individually
2. We shouldn't copy any code from GitLab

[fatkodima]

@ankane
In point 2, you mean due to licensing?
And what about mentioned changing activerecord requirement to >= 5.2?

[ankane]

Yeah, due to licensing (although it's also MIT, so we could use it), but mostly I'd rather recreate them from first principles in their simplest form to start.

Re activerecord: we shouldn't change the version requirements, but we can have some helpers only work with AR 5.2+ (and raise an error message for earlier versions) if it's significantly more work to make them work with earlier versions.

[ankane]

Closing in favor of separate PRs. @fatkodima, would love to get your thoughts on #111 when you have a chance.

[ankane]

Also, fwiw, I made the helpers opt-in on master to demonstrate what that would look like.