WL#926 "AVG(DISTINCT) and other distincts", part 2 (out of 3): clean up

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

sql/sql_class.h

@@ -1831,6 +1831,7 @@ public:
   Unique(qsort_cmp2 comp_func, void *comp_func_fixed_arg,
 	 uint size_arg, ulong max_in_memory_size_arg);
   ~Unique();
+  ulong elements_in_tree() { return tree.elements_in_tree; }
   inline bool unique_add(void *ptr)
   {
     DBUG_ENTER("unique_add");