Convert a synapse.events to async/await. (#7949)

changelog.d/7948.misc

changelog.d/7949.misc

changelog.d/7951.misc

synapse/api/auth.py

synapse/events/builder.py

synapse/events/snapshot.py

synapse/events/third_party_rules.py

synapse/events/utils.py

synapse/handlers/federation.py

synapse/replication/http/federation.py

synapse/replication/http/send_event.py

tests/storage/test_redaction.py

tests/test_state.py