Demonstrate workflow.all_handlers_finished

[drewhoskins-temporal]

What was changed

Before workflow.all_handlers_finished existed, I worked around it by checking a lock to see if we were in the middle of any handlers before continuing as new.
Now in 1.7, we can use the new method to prevent dangling handlers.

Why?

Demonstrate what should become standard practice.

Checklist

1. Closes

2. How was this tested:

# Terminal 1
cd updates_and_signals/safe_message_handlers
poetry run python ./worker.py
# Terminal 2
cd updates_and_signals/safe_message_handlers
poetry run python starter.py --test-continue-as-new

3. Any docs updates needed?

Yes, see the README.md

[dandavison]

LGTM