Fix customConfig race condition

[jacekbogdanski]

What is the purpose of this pull request?

Bug fix

Does your PR contain necessary tests?

All patches that change the editor code must include tests. You can always read more
on PR testing,
how to set the testing environment and
how to create tests
in the official CKEditor documentation.

This PR contains

• Unit tests
• Manual tests

Did you follow the CKEditor 4 code style guide?

Your code should follow the guidelines from the CKEditor 4 code style guide which helps keep the entire codebase consistent.

• PR is consistent with the code style guide

What is the proposed changelog entry for this pull request?

* [#3361](https://github.com/ckeditor/ckeditor4/issues/3361): Fixed: Loading multiple [custom editor configurations](https://ckeditor.com/docs/ckeditor4/latest/api/CKEDITOR_config.html#cfg-customConfig) is prone to race condition between them.

What changes did you make?

Honestly, I was really wondering about rewriting the whole customConfig loading as it could be nicely simplified and the current implementation is pretty bloated. Although, it's not a very good idea to do with a core code 10 years old. I propose simple fix instead, making sure that we won't overwrite cached config functor by a closure race condition.

Which issues does your PR resolve?

Closes #3361.

[Dumluregn]

Looks good, very simple and clear fix 👍 Unit and manual tests pass on all browsers too. I only proposed a few minor amendments.

[Dumluregn]

LGTM! 🎉