Cleanup the BufferLog when deleting an UprProducer

Change-Id: I4aab61c5133a7b5fe77e5575d4ca65964d918d75
Reviewed-on: http://review.couchbase.org/37975
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>