Item_sum_count_distinct, and
deploy Unique for use with COUNT(DISTINCT) if there is no blob
column in the list of DISTINCT arguments.


mysql-test/r/count_distinct2.result:
  Test results fixed.
mysql-test/r/func_group.result:
  Updated.
mysql-test/r/sum_distinct.result:
  Updated.
mysql-test/t/func_group.test:
  Add a test for COUNT(DISTINCT) and true varchar and case-insensitive
  collation. The table in the test contains only two distinct values.
mysql-test/t/sum_distinct.test:
  Since now we support INSERT INTO t1 (a) SELECT a+1 FROM t1, shorten
  the test.
  Add a nominal test for AVG(DISTINCT)
sql/item_sum.cc:
  Implementation of cleaned up Item_sum_count_distinct.
  Fixed a bug with COUNT(DISTINCT) and new VARCHAR and collations.
  Fixed a bug wiht AVG(DISTINCT) and wrong number of output digits
  after decimal point.
sql/item_sum.h:
  Cleanup for Item_sum_count_distinct.
  Now if the list of distinct arguments doesn't contain a blob column,
  we always use Unique and merge-sort to find distinct values.
sql/sql_class.h:
  Added a short-cut to find number of elements in Unique if all elements fit
  into memory.