Support multiple handlers of the same kind

[lemonsaurus]

Right now, we can only support one of each handler type - but what if you have two postgres databases in completely different places?

Let's find some way to dynamically instantiate one handler per connstring. For example, we could move the connstring parser out of the mixin, and then instantiate the handler with a factory method where we pass in the connstring. This would probably be tidier and less magical, anyway.