Raise notes if indexes cannot be used:
- in case of data type or collation mismatch (diferent error messages).
- in case if a table field was replaced to something else
  (e.g. Item_func_conv_charset) during a condition rewrite.

Added option to write warnings and notes to the slow query log for
slow queries.

New variables added/changed:

- note_verbosity, with is a set of the following options:
  basic            - All old notes
  unusable_keys    - Print warnings about keys that cannot be used
                     for select, delete or update.
  explain          - Print unusable_keys warnings for EXPLAIN querys.

The default is 'basic,explain'. This means that for old installations
the only notable new behavior is that one will get notes about
unusable keys when one does an EXPLAIN for a query. One can turn all
of all notes by either setting note_verbosity to "" or setting sql_notes=0.

- log_slow_verbosity has a new option 'warnings'. If this is set
  then warnings and notes generated are printed in the slow query log
  (up to log_slow_max_warnings times per statement).

- log_slow_max_warnings   - Max number of warnings written to
                            slow query log.

Other things:
- One can now use =ALL for any 'set' variable to set all options at once.
  For example using "note_verbosity=ALL" in a config file or
  "SET @@note_verbosity=ALL' in SQL.
- mysqldump will in the future use @@note_verbosity=""' instead of
  @sql_notes=0 to disable notes.
- Added "enum class Data_type_compatibility" and changing the return type
  of all Field::can_optimize*() methods from "bool" to this new data type.

Reviewer & Co-author: Alexander Barkov <bar@mariadb.com>
- The code that prints out the notes comes mainly from Alexander