fixed bug/updated count_distinct2 test
changed reset in count distinct to avoid calls to my_free()


include/my_sys.h:
  option to free_root() not to do my_free()
include/my_tree.h:
  reset_tree()
mysql-test/r/count_distinct2.result:
  added group by test
mysql-test/t/count_distinct2.test:
  group by test + fixed bug - need to drop table
mysys/my_alloc.c:
  mark_blocks_free()
mysys/tree.c:
  reset_tree()
sql/item_sum.cc:
  in count distinct reset_tree instead of delete_tree