Prior to this change foreign key constraint references could be left pointing
at tables dropped during operations simulating unsupported table alterations
because of an unexpected failure to disable foreign key constraint checks.
SQLite3 does not allow disabling such checks while in a transaction so they
must be disabled beforehand.
Thanks ezaquarii for the report and Carlton and Tim for the review.
SQLite 3.26 repoints foreign key constraints on table renames even when
foreign_keys pragma is off which breaks every operation that requires
a table rebuild to simulate unsupported ALTER TABLE statements.
The newly introduced legacy_alter_table pragma disables this behavior
and restores the previous schema editor assumptions.
Thanks Florian Apolloner, Christoph Trassl, Chris Lamb for the report and
troubleshooting assistance.
