Fixing a couple of small bugs with the multi device iterator having to deal

with the case when the background thread terminated because the iterator
finished and yet some other requests were coming in.

1. The GetNextFromShard would see an empty buffer and return cancelled instead
of OutOfRange errors
2. On shutdown, we weren't calling all the pending callbacks.

Tested with runs_per_test=5000

PiperOrigin-RevId: 214939274