Update generator stop yield when fetching history.

[RuofengX]

I have tow thread/coroutine in my app:

1. One is listening every update from all joined groups/channels, use while let Ok(...) = client.next_update().await.
2. The other is rolling-fetch all joined groups and channels, around 4000 messages per second. use let mut history = client.iter_messages(&packed_chat);

The issue is, when thread 2 start running, thread 1 stop yield. When thread 2 pause for a few minutes, thread 1 still not recovery, acting like dead thread.