AO3-6893 Use rebuild_constraints to modify foreign keys

[redsummernight]

Issue

https://otwarchive.atlassian.net/browse/AO3-6893

Purpose

@zz9pzza referenced this blog post. Our options:

• rebuild_constraints is safest but slow since it involves copying data from the old to the new table.
• drop_swap is fast but risky and not rollback friendly.
• auto is not a real third option, it picks between the first two by estimating how long it takes to copy data between the two tables.

Since we're using MariaDB, rebuilding constraints as an online operation can be fast (not requiring a full table copy) as long as we disable foreign_key_checks:

InnoDB supports adding foreign key constraints to a table with ALGORITHM set to INPLACE. In order to add a new foreign key constraint to a table with ALGORITHM set to INPLACE, the foreign_key_checks system variable needs to be set to OFF. If it is set to ON, then ALGORITHM=COPY is required.

This operation only changes the table's metadata, so the table does not have to be rebuilt.

We would need to set:

--alter-foreign-keys-method=rebuild_constraints
--set-vars=foreign_key_checks=0

However, the version of pt-osc we use (3.5.4) has percona/percona-toolkit#483, which set foreign_key_checks=0 only briefly when running alter table. That means we can simply set:

--alter-foreign-keys-method=rebuild_constraints

Testing Instructions

Retry the migrations in #5039.