Allow background tasks to be run on a separate worker. (#8369)

changelog.d/8369.feature

docs/sample_config.yaml

docs/workers.md

synapse/app/_base.py

synapse/app/admin_cmd.py

synapse/app/generic_worker.py

synapse/app/homeserver.py

synapse/app/phone_stats_home.py

synapse/config/workers.py

synapse/handlers/auth.py

synapse/handlers/stats.py

synapse/server.py

synapse/storage/databases/main/__init__.py

synapse/storage/databases/main/metrics.py

synapse/storage/databases/main/monthly_active_users.py

synapse/storage/databases/main/room.py

synapse/storage/databases/main/ui_auth.py

tests/test_phone_home.py

tests/utils.py