yield for each batch_size in for fixing audit logs

currently, we try and get batch_size based just on the last key seen.
but then we have to make sure we get all records for that key, iterating
on last ordered_id seen for that key. this is correct for covering the
keyspace completely, but some keys may have several batches worth of
ordered_ids. yield for each iteration of the second tier iteration as
well.

Change-Id: Idfc23ef650de63d6afd7a1787e9061a558afa892
Reviewed-on: https://gerrit.instructure.com/38661
Reviewed-by: Nick Cloward <ncloward@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Jacob Fugal <jacob@instructure.com>
QA-Review: Jacob Fugal <jacob@instructure.com>