Convert additional database code to async/await. (#8195)

changelog.d/8195.misc

synapse/appservice/__init__.py

synapse/federation/persistence.py

synapse/handlers/federation.py

synapse/storage/databases/main/appservice.py

synapse/storage/databases/main/deviceinbox.py

synapse/storage/databases/main/e2e_room_keys.py

synapse/storage/databases/main/event_federation.py

synapse/storage/databases/main/group_server.py

synapse/storage/databases/main/keys.py

synapse/storage/databases/main/transactions.py