We had a couple calls that used the default database, breaking

operations when testing against multiple databases. This involved calls

to get_app_upgrade_info() in signature deserialization (which didn't

know about databases) and the saving of Version and Evolution

models.
This updates all signature deserialization to support specifying a

database, and updates the model saving to use the evolver's assigned

database.

Unit tests pass on all supported versions of Django.