BKDWriter refactor: Simplify BKDWriter class [LUCENE-9291]

This is an umbrella issue for an effort to simplify the `BKDWriter` class.

The motivation behind this change is that the current class is doing quite a lot of things and it has become very difficult to read and therefore it is actually pretty difficult to introduce new optimisations / changes.

Anther motivation for this change is to remove the class `SimpleTextBKDWriter` which is a fork of the previous class and it is a burden to maintain.

I am dividing this refactor in four PR so it is easy to review. I will open shortly the Jira issues for each change and link them in this issue.

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Migrated from [LUCENE-9291](https://issues.apache.org/jira/browse/LUCENE-9291) by Ignacio Vera (@iverase)