MB-9919: expose DbInfo.file_size via couchstore_db_info

Change-Id: Iae8c76b7bf6b1a305be76b401f383a47766241e5
Reviewed-on: http://review.couchbase.org/32468
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>