backend, logger: lock before reassigning defaultBackend

[Asalle]

If not locked, this results in race:

1. SetBackend(), backend.go:26 - reassigning the global variable (write)

func SetBackend(backends ...Backend) LeveledBackend {
	var backend Backend
  // ...

  // DARA RACE START
	defaultBackend = AddModuleLevel(backend)
  // DATA RACE END

	return defaultBackend
}

2. IsEnabledFor(), logger.go:141 - accessing a default variable (read)

func (l *Logger) IsEnabledFor(level Level) bool {
	return defaultBackend.IsEnabledFor(level, l.Module) // DATA RACE HERE
}