Use sender+timestamp to cache receipts and read syncs

ts/messageModifiers/generateCacheKey.ts

@@ -0,0 +1,16 @@
+// Copyright 2016 Signal Messenger, LLC
+// SPDX-License-Identifier: AGPL-3.0-only
+
+// This function is necessary because the only thing we can guarantee will be unique is
+//  three pieces of data: sender, deviceId, and timestamp.
+// Because we don't care which device interacted with our message, we collapse this down
+//  to: sender + timestamp.
+export function generateCacheKey({
+  sender,
+  timestamp,
+}: {
+  sender: string;
+  timestamp: number;
+}): string {
+  return `cacheKey-${sender}-${timestamp}`;
+}