selectMessagesUnit
inline suspend fun <T : MessageEvent> T.selectMessagesUnit(timeoutMillis: Long = -1, filterContext: Boolean = true, priority: EventPriority = EventPriority.MONITOR, crossinline selectBuilder: MessageSelectBuilderUnit<T, Unit>.() -> Unit)
selectMessages 的 Unit 返回值捷径 (由于 Kotlin 无法推断 Unit 类型)