Better support of empty directories (#5890)

Better support of HEAD and listing of zero sized objects with trailing
slash (a.k.a empty directory). For that, isLeafDir function is added
to indicate if the specified object is an empty directory or not. Each
backend (xl, fs) has the responsibility to store that information.
Currently, in both of XL & FS, an empty directory is represented by
an empty directory in the backend.

isLeafDir() checks if the given path is an empty directory or not,
since dir listing is costly if the latter contains too many objects,
readDirN() is added in this PR to list only N number of entries.
In isLeadDir(), we will only list one entry to check if a directory
is empty or not.

cmd/storage-rpc-server.go

@@ -120,7 +120,7 @@ func (s *storageServer) ListDirHandler(args *ListDirArgs, reply *[]string) error
 		return err
 	}
 
-	entries, err := s.storage.ListDir(args.Vol, args.Path)
+	entries, err := s.storage.ListDir(args.Vol, args.Path, args.Count)
 	if err != nil {
 		return err
 	}