Call ReferenceProcessorPhaseTimes::set_processing_is_mt once

`ReferenceProcessor::process_discovered_references` is structured like:

```
{
  // SoftWeakFinalRefsPhase
  set_processing_is_mt
  
  // KeepAliveFinalRefsPhase
  set_processing_is_mt
  
  // PhantomRefsPhase
  set_processing_is_mt
}
``

Instead of calling it in each subphase, one can call it on the top level, because the original value of `_num_queues` is restored at the end of each subphase.