Histogram code cleanup and fixes

Factor the code that updates count, count_distinct,
count_distinct_single_occurrence into class Basic_stats_collector

Change from Histogram_builder and its descendant Histogram_builder_json
to  Histogram_builder (the interface), and Histogram_binary_builder,
Histogram_json_builder.

In Histogram_json_builder, do not forget to collect the right bound
of the right-most bucket.

sql/sql_statistics.h

@@ -403,7 +403,8 @@ public:
     return size;
   }
 
-  void init_for_collection(MEM_ROOT *mem_root, Histogram_type htype_arg, ulonglong size) override;
+  void init_for_collection(MEM_ROOT *mem_root, Histogram_type htype_arg,
+                           ulonglong size) override;
 
   bool is_available() override {return true; }