In MySQL 8.0, for InnoDB tables, name of key specified in FOREIGN KEY <name>
part of foreign key definition was used as a name of the foreign key created
in cases when there was no name specified using CONSTRAINT <name>
clause. In MySQL 5.7 the name of foreign key was automatically generated
in the same situation.

Such a change in behavior this has broken user applications which relied on
the fact that it was possible to create several foreign keys using the same
name in FOREIGN KEY <name> part of foreign key definition within the same
database. This also could have caused problems when replicating DDL adding/
dropping such foreign keys from MySQL 5.7 to 8.0.

Note that for NDB tables name from the FOREIGN KEY <name> part of
definition was used as foreign key name in absence of name in CONSTRAINT
clause in both 5.7 and 8.0.

This patch fixes the issue by changing 8.0 behavior for InnoDB tables
to be the same as in 5.7. This is an incompatible change for 8.0 branch,
which, for example, can break user applications written with old 8.0
behavior in mind or DDL replication from earlier version of 8.0.
However, we prefer to do it to restore compatibility with 5.7 and
fix cross-version replication from 5.7 branch.

Behavior for foreign keys on NDB tables is changed to be consistent with
InnoDB, i.e. to be the same as behavior for foreign keys on InnoDB tables
in 5.7. This is incompatible change, but it makes behavior uniform and
is much smaller than existing difference in foreign key name handling for
NDB tables between 5.7 and 8.0 (the latter requires foreign key names to
be unique within same schema and uses different format for auto-generated
foreign key names than 5.7).

Approved by: Sivert Sorumgaard <sivert.sorumgaard@oracle.com>