MB-8721 reduce log level on underflow error to INFO

reduce the underflow log level to INFO level if this happens only
during soft shutdown and bucket deletion

Change-Id: If5006f5da0952851e3a14f1a504019bf5ca7a5eb
Reviewed-on: http://review.couchbase.org/37462
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>