Fix a bug in position_in_interval(). Do not overwrite one interval endpoint
with another.

The problem was introduced in fix for MDEV-26724. That patch has made it
possible for histogram collection to fail. In particular, it fails for
non-assigned characters.

When histogram construction fails, we also abort the computation of
COUNT(DISTINCT). When we try to use the value, we get valgrind failures.

Switched the code to abort the statistics collection in this case.

When computing bucket_capacity= records/histogram->get_width(), round
the value UP, not down.

Part#3:
- make json_escape() return different errors on conversion error
  and on out-of-space condition.
- Make histogram code handle conversion errors.

Fix the description

Change it to LONGBLOB.
Also, update_statistics_for_table() should not "swallow" an error
from open_stat_tables.

.. part#2: correctly pass the charset to JSON [un]escape functions

MDEV-26595: ASAN use-after-poison my_strnxfrm_simple_internal / Histogram_json_hb::range_selectivity

Add testcase

Item_func_decode_histogram::val_str should correctly set null_value
when "decoding" JSON histogram.

Correctly handle empty string when [un]escaping JSON

Escape values when serializing to JSON. Un-escape when reading back.

Do not put Histogram objects on MEM_ROOT at all

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.