Sort class handler public declarations.

sql/handler.h:
  Collect ha_* wrappers and their virtual counterparts into two
  symmetrical groups. Previously they were scattered all over class
  handler declaration. That should ease maintenance and addition
  of new ha_ wrappers.
  When a new ha_* wrapper is added, it should be added to the
  end of the ha_* group. Its virtual counterpart should become private
  and be moved to the end of the hander class.