- Remove virtual from get_min_join_buffer_size() and
  get_max_join_buffer_size().
- Avoid some calls to get_min_buffer_size()
- Simply cache usage in get_..._join_buffer_size()
- Simplify get_max_join_buffer_size() when using optimize_buff_size
- Reindented some long comments

Reviewer: Sergei Petrunia <sergey@mariadb.com>