MDEV-15926 MEDIUMINT returns wrong I_S attributes

Problem:

The logic in store_column_type() with a switch on field type was
hard to follow. The part for MEDIUMINT (MYSQL_TYPE_INT24) was not correct.
It erroneously calculated the precision of MEDIUMINT UNSIGNED
as 7 instead of 8.

A similar hard-to-follow switch doing some type specific calculations
resided in adjust_max_effective_column_length(). It was also wrong for
MEDIUMINT (reported as a separate issue in MDEV-15946).

Solution:

1. Introducing a new class Information_schema_numeric_attributes
2. Adding a new virtual method Field::information_schema_numeric_attributes()
3. Splitting the logic in store_column_type() into virtual
   implementations of information_schema_numeric_attributes().
4. In order to avoid adding duplicate code for the integer data types,
   adding a new virtual method Field_int::numeric_precision(),
   which returns the number of digits.

Additional changes:

1. Adding the "const" qualifier to Field::max_display_length()

2. Moving the code from adjust_max_effective_column_length()
  directly to Field::max_display_length().
  There was no any sense to have two implementations:
  - a set of wrong virtual implementations for Field_xxx::max_display_length()
  - additional code in adjust_max_effective_column_length() fixing
    bad results of Field_xxx::max_display_length()
  This change is safe:
  - The code using Field::max_display_length()
    in field.cc, sql_show.cc, sql_type.cc is not affected.
  - The code in rpl_utility.cc is also not affected.
    See a new DBUG_ASSSERT and new comments explaining why.

  In the new reduction, Field_xxx::max_display_length() returns
  correct results for all integer types (except MEDIUMINT, see below).

  Putting implementations of numeric_precision() and max_display_length()
  near each other in field.h made the logic much clearer and thus
  helped to reveal bad results for Field_medium::max_display_length(),
  which returns 9 instead of 8 for signed MEDIUMINT fields.
  This problem will be addressed separately (MDEV-15946).

Note, this change is also useful for pluggable data types (see MDEV-4912),
as now a user defined Field_xxx has a way to control what's returned
in INFORMATION_SCHEMA.COLUMNS.NUMERIC_PRECISION and
INFORMATION_SCHEMA.COLUMNS.NUMERIC_SCALE by implementing
a desired behavior in Field_xxx::information_schema_numeric_attributes().

sql/rpl_utility.cc

@@ -42,6 +42,12 @@ max_display_length_for_temporal2_field(uint32 int_display_length,
    @param sql_type Type of the field
    @param metadata The metadata from the master for the field.
    @return Maximum length of the field in bytes.
+
+   The precise values calculated by field->max_display_length() and
+   calculated by max_display_length_for_field() can differ (by +1 or -1)
+   for integer data types (TINYINT, SMALLINT, MEDIUMINT, INT, BIGINT).
+   This slight difference is not important here, because we call
+   this function only for two *different* integer data types.
  */
 static uint32
 max_display_length_for_field(enum_field_types sql_type, unsigned int metadata)
@@ -737,6 +743,16 @@ can_convert_field_to(Field *field,
     case MYSQL_TYPE_INT24:
     case MYSQL_TYPE_LONG:
     case MYSQL_TYPE_LONGLONG:
+      /*
+        max_display_length_for_field() is not fully precise for the integer
+        data types. So its result cannot be compared to the result of
+        field->max_dispay_length() when the table field and the binlog field
+        are of the same type.
+        This code should eventually be rewritten not to use
+        compare_lengths(), to detect subtype/supetype relations
+        just using the type codes.
+      */
+      DBUG_ASSERT(source_type != field->real_type());
       *order_var= compare_lengths(field, source_type, metadata);
       DBUG_ASSERT(*order_var != 0);
       DBUG_RETURN(is_conversion_ok(*order_var, rli));