Change EventContext to use the Storage class (#6564)

changelog.d/6564.misc

synapse/api/auth.py

synapse/events/snapshot.py

synapse/events/third_party_rules.py

synapse/handlers/_base.py

synapse/handlers/federation.py

synapse/handlers/message.py

synapse/handlers/room.py

synapse/handlers/room_member.py

synapse/push/bulk_push_rule_evaluator.py

synapse/replication/http/federation.py

synapse/replication/http/send_event.py

synapse/storage/data_stores/main/push_rule.py

synapse/storage/data_stores/main/roommember.py

tests/test_state.py