Fix configuration handling bugs: (#5473)

* Update the GetConfig admin API to use the latest version of
  configuration, along with fixes to the corresponding RPCs.
* Remove mutex inside the configuration struct, and inside
  notification struct.
* Use global config mutex where needed.
* Add `serverConfig.ConfigDiff()` that provides a more granular diff
  of what is different between two configurations.

cmd/admin-handlers.go

@@ -795,7 +795,7 @@ func (a adminAPIHandlers) UpdateCredentialsHandler(w http.ResponseWriter,
 	}
 
 	// Take a lock on minio/config.json. Prevents concurrent
-	// config file/credentials updates.
+	// config file updates.
 	configLock := globalNSMutex.NewNSLock(minioReservedBucket, minioConfigFile)
 	if configLock.GetLock(globalObjectTimeout) != nil {
 		writeErrorResponseJSON(w, ErrOperationTimedOut, r.URL)
@@ -803,6 +803,10 @@ func (a adminAPIHandlers) UpdateCredentialsHandler(w http.ResponseWriter,
 	}
 	defer configLock.Unlock()
 
+	// Acquire lock before updating global configuration.
+	globalServerConfigMu.Lock()
+	defer globalServerConfigMu.Unlock()
+
 	// Notify all other Minio peers to update credentials
 	updateErrs := updateCredsOnPeers(creds)
 	for peer, err := range updateErrs {