Add option to move event persistence off master (#7517)

changelog.d/7517.feature

scripts/synapse_port_db

synapse/app/generic_worker.py

synapse/config/logger.py

synapse/config/workers.py

synapse/handlers/federation.py

synapse/handlers/message.py

synapse/handlers/presence.py

synapse/handlers/room.py

synapse/handlers/room_member.py

synapse/replication/http/__init__.py

synapse/replication/http/_base.py

synapse/replication/http/membership.py

synapse/replication/http/presence.py

synapse/replication/tcp/handler.py

synapse/rest/admin/rooms.py

synapse/storage/data_stores/__init__.py

synapse/storage/data_stores/main/devices.py

synapse/storage/data_stores/main/events.py

synapse/storage/data_stores/main/events_worker.py