Get rid of IteratorBase::is_exhausted flag since it is not possible to rely on...

Get rid of IteratorBase::is_exhausted flag since it is not possible to rely on it unless we lock each call to GetNext which is not preferable.
Each iterator now handles saving/restoring exhausted state.
As a guideline, we always reset the input_impl(s) when they get exhausted. This can be used as an indicator of exhausted-ness for non-terminal iterators. Also reduces memory overhead.
Each iterator should also handle calls to GetNextInternal when it is exhausted. Fixed this for some datasets.
Also fix a bug in dataset_serialization_test_base. We were not saving
a checkpoint after exhausting the iterator so verify_exhausted_iterator
was not really testing restoring an exhausted iterator.

PiperOrigin-RevId: 175253023