Return appropriate errors API versions changes across REST APIs (#8480)

This PR adds code to appropriately handle versioning issues
that come up quite constantly across our API changes. Currently
we were also routing our requests wrong which sort of made it
harder to write a consistent error handling code to appropriately
reject or honor requests.

This PR potentially fixes issues

 - old mc is used against new minio release which is incompatible
   returns an appropriate for client action.
 - any older servers talking to each other, report appropriate error
 - incompatible peer servers should report error and reject the calls
   with appropriate error

cmd/config.go

@@ -233,7 +233,8 @@ func (sys *ConfigSys) Init(objAPI ObjectLayer) error {
 		select {
 		case <-retryTimerCh:
 			if err := initConfig(objAPI); err != nil {
-				if strings.Contains(err.Error(), InsufficientReadQuorum{}.Error()) ||
+				if err == errDiskNotFound ||
+					strings.Contains(err.Error(), InsufficientReadQuorum{}.Error()) ||
 					strings.Contains(err.Error(), InsufficientWriteQuorum{}.Error()) {
 					logger.Info("Waiting for configuration to be initialized..")
 					continue
@@ -264,6 +265,19 @@ func initConfig(objAPI ObjectLayer) error {
 		}
 	}
 
+	// Construct path to config/transaction.lock for locking
+	transactionConfigPrefix := minioConfigPrefix + "/transaction.lock"
+
+	// Hold lock only by one server and let that server alone migrate
+	// all the config as necessary, this is to ensure that
+	// redundant locks are not held for each migration - this allows
+	// for a more predictable behavior while debugging.
+	objLock := globalNSMutex.NewNSLock(context.Background(), minioMetaBucket, transactionConfigPrefix)
+	if err := objLock.GetLock(globalOperationTimeout); err != nil {
+		return err
+	}
+	defer objLock.Unlock()
+
 	// Migrates ${HOME}/.minio/config.json or config.json.deprecated
 	// to '<export_path>/.minio.sys/config/config.json'
 	// ignore if the file doesn't exist.