Freeze models for healing migration

Take a snapshot of all models from the code base at the time when the
healing migration merges. The healing migration needs this frozen view
of the models to be available (even as the models change in the future)
to compare with the current DB schema. The healing migration will use
this comparison to heal the schema.

partially implement bp: db-migration-refactor

Change-Id: I438147c7961b1ecad47f6146cc7fc9396aee7bc5
Merge "Freeze models for healing migration"