VALUES FOR FOREIGN KEYS".

ALTER TABLE that changes a column definition may lead to changes
in values stored in this column. If such a column participates in
a foreign key (which is not added by this ALTER TABLE) changes in
the column might break referential integrity.

Same could have happened when ALTER IGNORE TABLE removed some
rows from the table that served as a parent in a foreign key.

This has happened because there was no check that for a new column
value there is a matching parent key value (or if parent table
was changed that such a change doesn't leave child key values
as orphans).

This patch tries to address the problem for the case when ALTER
TABLE is executed using COPY-algorithm. The corresponding problem
for the INPLACE algorithm should be handled inside of storage
engine calls implementing it.

Incompatible changes:
1) We no longer allow dropping or renaming of columns in a foreign
   key when ALTER is executed using COPY algorithm (Rename is
   disallowed since we miss an API to communicate information about
   new names of columns to the InnoDB engine).
2) We do not allow changes to parent and child columns which
   theoretically might result in value change. "Insignificant"
   changes that can't result in value change are still allowed.
3) An error is emitted when ALTER IGNORE TABLE tries to delete a
   row from a table which is a parent in a foreign key.

Checks 2-3) are not carried out when @@foreign_key_checks are off (=0).

Long-term we can allow "significant" changes to child columns.
To implement this we need a storage engine method that will temporarily
copy info about affected foreign keys from the old version of table
to the new version, so foreign key checks can be performed during copy.