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.

Author: spetrunia