S3Guard to self update on directory listings of S3

S3Guard updates its table on a getFileStatus call, but not on a directory listing.

While this makes directory listings faster (no need to push out an update), it slows down subsequent queries of the files, such as a sequence of:

statuses = s3a.listFiles(dir)
for (status: statuses) {
  if (status.isFile) {
      try(is = s3a.open(status.getPath())) {
        ... do something
      }
}

this is because the open() is doing the getFileStatus check, even after the listing.

Updating the DDB tables after a listing would give those reads a speedup, albeit at the expense of initiating a (bulk) update in the list call. Of course, we could consider making that async, though that design (essentially a write-buffer) would require the buffer to be checked in the reads too.