Provide buffer of sufficient size.

Aslo add more test coverage

- Use String::c_ptr_safe() instead of String::c_ptr
- Do proper datatype conversions in Histogram_json_hb::parse
- Remove Histogram_json_hb::Bucket::end_value. Introduce
  get_end_value() instead.

Basic ideas:
1. Store "popular" values in their own buckets.
2. Also store ndv (Number of Distinct Values) in each bucket.

Because of #1, the buckets are now variable-size, so store the size in
each bucket.

Adjust selectivity estimation functions accordingly.

- Fix bad tests in statistics_json test: make them meaningful and make them
  work on windows
- Fix analyze_debug.test: correctly handle errors during ANALYZE

Remove Histogram_*::is_available(), it is not applicable anymore.
Fix compilation on Windows

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.

There are "local" histograms that are allocated by one thread for
one TABLE object, and "global" that are allocated for TABLE_SHARE.

- Don't use 'res' uninitialized
- multiply it by col_non_nulls before set_if_bigger(...) call.

Signed-off-by: Michael Okoko <okokomichaels@outlook.com>

Signed-off-by: Michael Okoko <okokomichaels@outlook.com>