Message-processing design patterns

[dandavison]

This PR contains proposals for how the Python SDK could help users defer update processing, control interleaving of handler coroutines, and ensure processing is complete before workflow completion.

update/job_runner_I1.py and update/job_runner_I2.py show how users can do this themselves, with minimal SDK changes.

update/job_runner_I1_native.py and update/job_runner_I2_native.py show how the SDK could be modified to make this easier and less error-prone for users.