Update code to refer to "workers". (#15606)

changelog.d/15606.misc

docs/replication.md

synapse/app/admin_cmd.py

synapse/app/generic_worker.py

synapse/module_api/__init__.py

synapse/replication/tcp/client.py

synapse/storage/databases/main/account_data.py

synapse/storage/databases/main/cache.py

synapse/storage/databases/main/devices.py

synapse/storage/databases/main/events_worker.py

synapse/storage/databases/main/receipts.py

synapse/storage/schema/main/delta/34/cache_stream.py

tests/app/test_openid_listener.py

tests/replication/slave/storage/__init__.py

tests/replication/storage/__init__.py

tests/replication/storage/_base.py

tests/replication/storage/test_events.py