There was confusion about how to migrate data from third-party

applications when you are going to uninstall the application later on.

Thanks to Markus, Marten and Sergei for help and review.

Added the Migration.atomic attribute which can be set to False
for non-atomic migrations.

Thanks Markus Holtermann for review.

The new signature enables better support for routing RunPython and
RunSQL operations, especially w.r.t. reusable and third-party apps.

This commit also takes advantage of the deprecation cycle for the old
signature to remove the backward incompatibility introduced in #22583;
RunPython and RunSQL won't call allow_migrate() when when the router
has the old signature.

Thanks Aymeric Augustin and Tim Graham for helping shape up the patch.

Refs 22583.