Thus, all these variables will be
grouped together and more logically named.

Descriptions for the old variables were updated to indicate they are
now aliases for the newly introduced variables with prefix log_slow.

log_slow_queries_not_using_indexes_filter will not be addressed
in this merge request.
log_throttle_queries_not_using_indexes seems to no longer be in use.

MTR tests are also updated to include the new variable names.

All new code of the whole pull request, including one or several files
that are either new files or modified ones, are contributed under the
BSD-new license. I am contributing on behalf of my employer Amazon Web
Services, Inc.