Some non-unique constraint names were added in
b69f8eb04cc8762d3dfd5af5ea1fc58e3f2ebcc3 which resulted in failures
depending on the order in which tests were run.
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.
