in storehaus-algebra/src/main/scala/com/twitter/storehaus/algebra/BufferingStore.scala [62:67]
override def multiMerge[K1 <: K](kvs: Map[K1, V]): Map[K1, Future[Option[V]]] = {
// no lazy
val links: Map[K, PromiseLink[V]] = kvs.map { case (k1, v) => k1 -> PromiseLink(v) }(breakOut)
summer.put(links).foreach(mergeFlush)
kvs.map { case (k, _) => k -> links(k).promise }
}