Store and read foreign key info into/from FRM files

1. Introduce Foreign_key_io class which creates/parses binary stream containing foreign key structures. Referenced tables store there only hints about foreign tables (their db and name), they restore full info from the corresponding tables.

Foreign_key_io is stored under new EXTRA2_FOREIGN_KEY_INFO field in extra2 section of FRM file.

2. Modify mysql_prepare_create_table() to generate names for foreign keys. Until InnoDB storage of foreign keys is removed, FK names must be unique across the database: the FK name must be based on table name.

3. Keep stored data in sync on DDL changes. Referenced tables update their foreign hints after following operations on foreign tables:

• RENAME TABLE
• DROP TABLE
• CREATE TABLE
• ADD FOREIGN KEY
• DROP FOREIGN KEY

Foreign tables update their foreign info after following operations on referenced tables:

• RENAME TABLE
• RENAME COLUMN

4. To achieve 3. there must be ability to rewrite extra2 section of FRM file without full reparse. FRM binary is built from primary structures like HA_CREATE_INFO and cannot be built from TABLE_SHARE.

Use shadow write and rename like fast_alter_partition_table() does.