config: setter/getter for Notifier and Logger into its own struct. (#3721)

This is an attempt cleanup code and keep the top level config
functions simpler and easy to understand where as move the
notifier related code and logger setter/getter methods as part
of their own struct.

Locks are now held properly not globally by configMutex, but
instead as private variables.

Final fix for #3700

cmd/config-migrate.go

@@ -855,7 +855,10 @@ func migrateV12ToV13() error {
 	}
 
 	// Copy over fields from V12 into V13 config struct
-	srvConfig := &serverConfigV13{}
+	srvConfig := &serverConfigV13{
+		Logger: &logger{},
+		Notify: &notifier{},
+	}
 	srvConfig.Version = "13"
 	srvConfig.Credential = cv12.Credential
 	srvConfig.Region = cv12.Region