Add support for non-integer PKs

[jerivas]

This PR adds support to broadcast updates for models that don't use integer PKs, for example UUID or HashID. The biggest change is converting pks_in_queryset to a set of strings (instead of integers). This will support any type that can be reliably cast to a string.

[davish]

The code looks good to me, and the tests pass, but I'm wondering about how querysets and the ORM handle the different types. Does django not care if you pass in a string or an int for a CharField / AutoField? Will it coerce to the proper type before firing off the SQL query? I'll look into the docs a bit more but wanted to ask in case you had experience with this.

[jerivas]

I would suspect it works fine since the original tests are passing and their pks are being cast to strings as well

[jerivas]

@davish I just verified it and it works: Django will find instances by their PK even if it's passed as a string and the actual value is an int

[jerivas]

@davish I fixed a bug where retrieve messages were being sent for unrelated model instances (also added a test case)