Changes to serialization and client kwargs
- Change the signature of TASK_HANDLER to take **kwargs instead of kwargs.
- Hook model instance is serialized before TASK_HANDLER is called,
meaning that delete hook will also have access to the instance id.
The SERIALIZER setting no longer takes the "hook" argument.
- CLIENT_KWARGS can no longer be defined on the model. The CLIENT_KWARGS
setting is now a global definition, which will be updated with the
hook headers and serialized instance json. CLIENT_KWARGS also no longer
takes the "instance" argument.
- Fix typo "thead_task_handler" to "thread_task_handler"
