MDEV-21130: Histograms: use JSON as on-disk format

Preparation for handling different kinds of histograms:

- In Column_statistics, change "Histogram histogram" into
  "Histogram *histogram_".  This allows for different kinds
  of Histogram classes with virtual functions.

- [Almost] remove the usage of Histogram->set_values and
  Histogram->set_size. The code outside the histogram should
  not make any assumptions about what/how is stored in the Histogram.

- Introduce drafts of methods to read/save histograms to/from disk.

Author: spetrunia