* fix potential race on close * move validate inside * demote commit and cleanup in image.Store to non-exposed functions. * replace immediate image commit with delayed via Submit * minor: remove error logging, rename tests * minor: err wrapping, comments wording * clarify FileSystem.Save code * attempt to fix #584 by making submitted image commits on a half of TTL Co-authored-by: Dmitry Verkhoturov <paskal.07@gmail.com>