Worker: HTTP calls to Master are performed asynchronously & in a diff thread

packages/flamenco-worker-python/flamenco_worker/cli.py

packages/flamenco-worker-python/flamenco_worker/upstream.py

...co-worker-python/flamenco_worker/upstream_update_queue.py

packages/flamenco-worker-python/flamenco_worker/worker.py

packages/flamenco-worker-python/tests/mock_responses.py

packages/flamenco-worker-python/tests/test_coro_mock.py

...lamenco-worker-python/tests/test_upstream_update_queue.py

packages/flamenco-worker-python/tests/test_worker.py