This change adds generic parameters for ConcurrencyManager so it can be

used where we would expect a Manager.
This particular class also hasn't been useful since the very early days

of Django, since the base Manager.get_or_create implementation now does

exactly what this code does, preventing us from ever hitting the

exception handler. Because of that, I've removed the get_or_create

implementation entirely and marked the class as deprecated.

Ran unit tests.