Fix writing 'format.json' and make it atomic (#8296)

- Choose a unique uuid such that under situations of duplicate
  mounts we do not append to an existing json entry.
- Avoid AppendFile instead use WriteAll() to write the entire
  byte array atomically.

cmd/format-meta.go

@@ -20,9 +20,6 @@ package cmd
 const (
 	// Format config file carries backend format specific details.
 	formatConfigFile = "format.json"
-
-	// Format config tmp file carries backend format.
-	formatConfigFileTmp = "format.json.tmp"
 )
 
 const (