Enable mypy checking for unreachable code and fix instances. (#8432)

changelog.d/8432.misc

mypy.ini

synapse/config/tls.py

synapse/federation/federation_server.py

synapse/handlers/directory.py

synapse/handlers/room.py

synapse/handlers/room_member.py

synapse/handlers/sync.py

synapse/http/server.py

synapse/logging/_structured.py

synapse/push/push_rule_evaluator.py

synapse/replication/tcp/protocol.py

synapse/state/__init__.py

synapse/storage/databases/main/censor_events.py

synapse/storage/databases/main/events.py

synapse/storage/databases/main/stream.py

synapse/storage/util/id_generators.py