The value of notification_count in unread_notifications returned by "/sync" is wrong

*This issue was originally created by [**@MyYuan**](https://github.com/MyYuan) at .*

Before sending a message, make a /sync request. If there are no new messages, the request will block and not return. At this time, another user sends a new message through the /send request, and the notification_count value in the unread_notifications returned by the /sync interface is wrong.

As shown below:

Returns correctly, notification_count should be equal to 1, not 0

Start at the /sync entry point and trace how unread_notifications and notification_count are calculated while a request is blocked. Reproduce the sequence with no new messages, a waiting /sync, and another user's /send request, then verify that the returned notification_count is 1 rather than 0.

Written by the indexing model from the issue text.