Summary
-------

Make the option `ParallelRefProcEnabled` to default to `true` for Parallel GC. Make reference processing for Parallel GC use the same heuristics as for G1 to calculate the number of threads to use for this phase.

Problem
-------

Today, the behaviour of G1 and Parallel GC differs in that in G1 the option `ParallelRefProcEnabled` is `true` by default and uses some heuristics to set the amount of  worker threads depending on the actually observed reference count. Neither is currently true for Parallel GC: the default for `ParallelRefProcEnabled` is `false`, and if the user manually enables it, all available gc worker threads will be used.

Solution
--------

Set `ParallelRefProcEnabled` to `true` by default and remove the special handling in Parallel GC so that it also uses the same ergonomic calculations as G1 for thread counts.

This change is positive in two ways: First it will make both garbage collectors use the same defaults and heuristics, second it will by default increase performance for Parallel GC when processing many references.

Specification
-------------

    diff --git a/src/hotspot/share/gc/parallel/parallelArguments.cpp b/src/hotspot/share/gc/parallel/parallelArguments.cpp
    index 185d2266796..2ffffdf1455 100644
    --- a/src/hotspot/share/gc/parallel/parallelArguments.cpp
    +++ b/src/hotspot/share/gc/parallel/parallelArguments.cpp
    @@ -85,6 +85,10 @@ void ParallelArguments::initialize() {
       if (FLAG_IS_DEFAULT(MarkSweepDeadRatio)) {
         FLAG_SET_DEFAULT(MarkSweepDeadRatio, 1);
       }
    +
    +  if (FLAG_IS_DEFAULT(ParallelRefProcEnabled) && ParallelGCThreads > 1) {
    +    FLAG_SET_DEFAULT(ParallelRefProcEnabled, true);
    +  }
     }
     
     // The alignment used for boundary between young gen and old gen