Bug#30216864 INFORMATION_SCHEMA.KEY_COLUMN_USAGE PERFORMANCE IS SLOW

Problem:
The system views INFORMATION_SCHEMA.KEY_COLUMN_USAGE and
INFORMATION_SCHEMA.TABLE_CONSTRAINTS uses UNION. This causes the
conditions to be evaluated once per each SELECT statement which
are part of UNION.  This causes dictionary metadata to be
traversed multiple times. Mainly the metadata tables mysql.schema
and mysql.tables would be traversed multiple times. And this
intern causes the access checks for SQL objects to be done
multiple times.

Fix:
Rewrite the system views for INFORMATION_SCHEMA.KEY_COLUMN_USAGE
and INFORMATION_SCHEMA.TABLE_CONSTRAINTS to move UNION into a
LATERAL table. That way, it is be possible to merge the view and
the conditions on schema name and table name is applied before
the lateral table is executed, enabling indexes of the specified
table to be accessed.

There is following change in behavior with this patch. The column
names of two INFORMATION_SCHEMA tables would be presented as
capitalized column names and not the case provided by user in the
query.

E.g.,
  SELECT Table_Name, Constraint_name
    FROM INFORMATION_SCHEMA.TABLE_CONSTRAINTS (or KEY_COLUMN_USAGE) LIMIT 1;
  TABLE_NAME      CONSTRAINT_NAME
  ...

  Unlike, the behavior before patch is that we get user provided case,

  SELECT Table_Name, Constraint_name
    FROM INFORMATION_SCHEMA.TABLE_CONSTRAINTS (or KEY_COLUMN_USAGE) LIMIT 1;
  Table_Name      Constraint_name
  ...

The new behavior is same as seen in other INFORMATION_SCHEMA
tables based on system view. E.g., I_S.TABLES, I_S.SCHEMATA.
See bug Bug#31195099 for more details.

Change-Id: Ie8d309024316b694c210fbb660d55d4eb7afa05e