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().

mysql-test/suite/funcs_1/r/is_columns_innodb.result

@@ -427,9 +427,9 @@ def	test	tb1	f19	19	NULL	YES	smallint	NULL	NULL	5	0	NULL	NULL	NULL	smallint(5) u
 def	test	tb1	f2	2	NULL	YES	char	0	0	NULL	NULL	NULL	latin1	latin1_bin	char(0)			select,insert,update,references		NEVER	NULL
 def	test	tb1	f20	20	NULL	YES	smallint	NULL	NULL	5	0	NULL	NULL	NULL	smallint(5) unsigned zerofill			select,insert,update,references		NEVER	NULL
 def	test	tb1	f21	21	NULL	YES	mediumint	NULL	NULL	7	0	NULL	NULL	NULL	mediumint(9)			select,insert,update,references		NEVER	NULL
-def	test	tb1	f22	22	NULL	YES	mediumint	NULL	NULL	7	0	NULL	NULL	NULL	mediumint(8) unsigned			select,insert,update,references		NEVER	NULL
-def	test	tb1	f23	23	NULL	YES	mediumint	NULL	NULL	7	0	NULL	NULL	NULL	mediumint(8) unsigned zerofill			select,insert,update,references		NEVER	NULL
-def	test	tb1	f24	24	NULL	YES	mediumint	NULL	NULL	7	0	NULL	NULL	NULL	mediumint(8) unsigned zerofill			select,insert,update,references		NEVER	NULL
+def	test	tb1	f22	22	NULL	YES	mediumint	NULL	NULL	8	0	NULL	NULL	NULL	mediumint(8) unsigned			select,insert,update,references		NEVER	NULL
+def	test	tb1	f23	23	NULL	YES	mediumint	NULL	NULL	8	0	NULL	NULL	NULL	mediumint(8) unsigned zerofill			select,insert,update,references		NEVER	NULL
+def	test	tb1	f24	24	NULL	YES	mediumint	NULL	NULL	8	0	NULL	NULL	NULL	mediumint(8) unsigned zerofill			select,insert,update,references		NEVER	NULL
 def	test	tb1	f25	25	NULL	YES	int	NULL	NULL	10	0	NULL	NULL	NULL	int(11)			select,insert,update,references		NEVER	NULL
 def	test	tb1	f26	26	NULL	YES	int	NULL	NULL	10	0	NULL	NULL	NULL	int(10) unsigned			select,insert,update,references		NEVER	NULL
 def	test	tb1	f27	27	NULL	YES	int	NULL	NULL	10	0	NULL	NULL	NULL	int(10) unsigned zerofill			select,insert,update,references		NEVER	NULL
@@ -543,9 +543,9 @@ def	test	tb3	f135	18	999	NO	smallint	NULL	NULL	5	0	NULL	NULL	NULL	smallint(5) un
 def	test	tb3	f136	19	00999	NO	smallint	NULL	NULL	5	0	NULL	NULL	NULL	smallint(5) unsigned zerofill			select,insert,update,references		NEVER	NULL
 def	test	tb3	f137	20	00999	NO	smallint	NULL	NULL	5	0	NULL	NULL	NULL	smallint(5) unsigned zerofill			select,insert,update,references		NEVER	NULL
 def	test	tb3	f138	21	9999	NO	mediumint	NULL	NULL	7	0	NULL	NULL	NULL	mediumint(9)			select,insert,update,references		NEVER	NULL
-def	test	tb3	f139	22	9999	NO	mediumint	NULL	NULL	7	0	NULL	NULL	NULL	mediumint(8) unsigned			select,insert,update,references		NEVER	NULL
-def	test	tb3	f140	23	00009999	NO	mediumint	NULL	NULL	7	0	NULL	NULL	NULL	mediumint(8) unsigned zerofill			select,insert,update,references		NEVER	NULL
-def	test	tb3	f141	24	00009999	NO	mediumint	NULL	NULL	7	0	NULL	NULL	NULL	mediumint(8) unsigned zerofill			select,insert,update,references		NEVER	NULL
+def	test	tb3	f139	22	9999	NO	mediumint	NULL	NULL	8	0	NULL	NULL	NULL	mediumint(8) unsigned			select,insert,update,references		NEVER	NULL
+def	test	tb3	f140	23	00009999	NO	mediumint	NULL	NULL	8	0	NULL	NULL	NULL	mediumint(8) unsigned zerofill			select,insert,update,references		NEVER	NULL
+def	test	tb3	f141	24	00009999	NO	mediumint	NULL	NULL	8	0	NULL	NULL	NULL	mediumint(8) unsigned zerofill			select,insert,update,references		NEVER	NULL
 def	test	tb3	f142	25	99999	NO	int	NULL	NULL	10	0	NULL	NULL	NULL	int(11)			select,insert,update,references		NEVER	NULL
 def	test	tb3	f143	26	99999	NO	int	NULL	NULL	10	0	NULL	NULL	NULL	int(10) unsigned			select,insert,update,references		NEVER	NULL
 def	test	tb3	f144	27	0000099999	NO	int	NULL	NULL	10	0	NULL	NULL	NULL	int(10) unsigned zerofill			select,insert,update,references		NEVER	NULL