Semantically rename reference processing subphases

Followup of JDK-8051680, use semantic names instead of phase plus number suffix.

From
```
  enum RefProcSubPhases {
    SoftRefSubPhase1,
    WeakRefSubPhase1,
    FinalRefSubPhase1,
    FinalRefSubPhase2,
    PhantomRefSubPhase3,
    RefSubPhaseMax
  };

  enum RefProcPhases {
    RefPhase1,
    RefPhase2,
    RefPhase3,
    RefPhaseMax
  };
```

to names like:

```
Process{Soft,Weak,Final}RefsSubPhase, KeepAliveFinalRefsSubPhase, ProcessPhantomRefsSubPhase

SoftWeakFinalRefsPhase, KeepAliveFinalRefsPhase, PhantomRefsPhase
```