Decrement session counter if compaction fails

Change-Id: Ia4379958e262161f2611097833b8de03b9494a8d
Reviewed-on: http://review.couchbase.org/39861
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>