Clear the checkpoint manager when doing partial rollback

The checkpoint manager might contain items that were rolled
back. As a result we should remove all items from the checkpoint
manager in order to make sure that we don't accidentally keep
and rolled back data.

Change-Id: I12ed857da0c000bda5a69e2182a6963c74536b3c
Reviewed-on: http://review.couchbase.org/38443
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>