management command update_index: Use last seen max pk for selecting batch starting point.
management command update_index: Use last seen max pk for selecting batch starting point.

This fixes (or at least mitigates) issue #1077 for the synchronous update case.

celerityweb/django-haystack,sgaist/django-haystack,celerityweb/django-haystack,sgaist/django-haystack,celerityweb/django-haystack,sgaist/django-haystack