Handle multiple tenant-aware database connections

[JamesMaroney]

I have some special requirements that cause me to have multiple defined database connections to the same tenant schemas (just with different postgres users). For this to work out properly, I had to beef up some of the method in utils.py to operate on django.db.connections instead of django.db.connection.

I tried to make sure that the updates are backwards compatible, but am open fixing anything I may have missed.

Let me know if there are any updates to documentation or tests that you'd like to see before considering this Pull Request. I'm assuming you'd want some documentation updates to cover the updated utils behavior.