Allow client event serialization to be async

synapse/events/utils.py

synapse/handlers/events.py

synapse/handlers/initial_sync.py

synapse/handlers/message.py

synapse/handlers/pagination.py

synapse/handlers/search.py

synapse/rest/client/v1/events.py

synapse/rest/client/v1/room.py

synapse/rest/client/v2_alpha/notifications.py

synapse/rest/client/v2_alpha/sync.py

synapse/server.py

synapse/util/async_helpers.py