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/logger-console-hook.go

@@ -26,7 +26,7 @@ type consoleLogger struct {
 
 // enable console logger.
 func enableConsoleLogger() {
-	clogger := serverConfig.GetConsoleLogger()
+	clogger := serverConfig.Logger.GetConsole()
 	if !clogger.Enable {
 		return
 	}