Fixed an infinite recursion bug when deleting models related to a tracked model with a ForeignKey

[nick-prat]

When deleting a model, that has a reverse relationship to a tracked model with SET_NULL on_delete

Problem

This issue arrises on these conditions:

1. You delete a model that is target of a foreign key, with on_delete=SET_NULL (I believe also with PROTECT)
2. The related model that is going to be deleted has a field tracker that tracks a foreign key field
   When this happens, django loads the instance from the db with all fields deferred other than id. In this code,

def current(self, fields=None):
    if fields is None:
        deferred_fields = self.deferred_fields
        if deferred_fields:
            fields = [
                field for field in self.fields
                if field not in deferred_fields
            ]
        else:
            fields = self.fields
    
    return {f: self.get_field_value(f) for f in fields}

we skip those deferred fields, but this is where the issue arrises, we don't check self.field_map[field] here, so even though the FK is deferred, we still attemp to load it, as we check for, as an example, foo, while the deffered field is foo_id.

Issue talked about here as well: #533