processEvent
abstract suspend fun processEvent(eventEnvelope: EventEnvelope): <Error class: unknown class><<Error class: unknown class>, Unit>
Process the eventEnvelope, persisting the last processed event ID if the event is not transient (see EventDeliveryInfo.isTransient). If the processing fails, the last processed event ID will not be updated.
Return
CoreFailure if the event processing failed, or Unit if the event was processed successfully.