RFR(XS): JDK-8212122: Allow ReferenceProcessor to always be MT processing

[sangheon.kim at oracle.com]

> Maybe the selection of whether to run the 1-thread case directly or
> via the executor should be a property of the executor?  That seems to
> be kind of what you are proposing, except I think you are suggesting
> the executor's execute function be responsible for that.  I think that
> didn't work for CMS and Parallel? Sangheon probably remembers better
> than exactly what the issues were there.
1. CMS has different caller side setting for single-multi thread cases. 
( more details at https://bugs.openjdk.java.net/browse/JDK-6938732 )
2. The reason for Parallel gc is a bit different. As it is not using 
WorkGang, changes are not trivial so it is not included at JDK-8043575. 
For this we have https://bugs.openjdk.java.net/browse/JDK-8204686.

Thanks,
Sangheon


> The choice could instead be
> a predicate on the executor, called with the number of threads.