queueMarkAsRead: Set timeout if there is mark request in less than 2s.

If there are multiple requests to mark messages as read with interval
less than 2s, and if there is no further request then later were
stuck in the queue. Becuase there was only one caller to
`messagesFlags`, which was only called by user scroll event.
So even after reaching at the end of message list, unread banner is
visible with some unread count.

So now set timeout to send read message flag to server.

Fixes: zulip#3509

src/api/queueMarkAsRead.js

@@ -2,15 +2,26 @@
 import type { Auth } from './transportTypes';
 import messagesFlags from './messages/messagesFlags';
 
+const TIME_INTERVAL_BETWEEN_CONSECUTIVE_CALLS_MS = 2000;
 let unsentMessageIds = [];
 let lastSentTime = 0;
+let timeout = null;
+
+const updateMessageFlags = (auth: Auth) => {
+  messagesFlags(auth, unsentMessageIds, 'add', 'read');
+  unsentMessageIds = [];
+  lastSentTime = Date.now();
+};
 
 export default (auth: Auth, messageIds: number[]): void => {
   unsentMessageIds.push(...messageIds);
 
-  if (Date.now() - lastSentTime > 2000) {
-    messagesFlags(auth, unsentMessageIds, 'add', 'read');
-    unsentMessageIds = [];
-    lastSentTime = Date.now();
+  if (Date.now() - lastSentTime > TIME_INTERVAL_BETWEEN_CONSECUTIVE_CALLS_MS) {
+    updateMessageFlags(auth);
+  } else if (timeout === null) {
+    timeout = setTimeout(() => {
+      updateMessageFlags(auth);
+      timeout = null;
+    }, TIME_INTERVAL_BETWEEN_CONSECUTIVE_CALLS_MS);
   }
 };