The name of the bulk thread pool was renamed to "write" with "bulk" as a
fallback name. This change was made in 6.x for BWC reasons yet in 7.0.0
we are removing this fallback. This commit removes this fallback for the
write thread pool.
* Add a CHANGELOG file for 7.x release notes.
* update file to include 6.x
* remove confusing comment and small edit to section title
* moving CHANGELOG file under docs directory, as it pertains to release notes.