During the `cagg_migrate` execution if the user set the `drop_old`
parameter to `true` the routine will drop the old Continuous Aggregate
leading to an inconsistent state because the catalog code don't handle
this table as a normal catalog table so the records are not removed
when dropping a Continuous Aggregate. The same problem will happen if
you manually drop the old Continuous Aggregate after the migration.
Fixed it by removing the useless Foreign Key and also adding another
column named `user_view_definition` to the main plan table just to store
the original user view definition for troubleshooting purposes.
Fixed#5662
f250eaa631