RFR: Missing Evac OOM scope when reference processor is single threaded

[Zhengyu Gu]

When ReferenceProcessor processes references in single-threaded mode, it 
uses current thread, not task executor, and we miss Evac OOM scope for 
current thread.

Webrev: 
http://cr.openjdk.java.net/~zgu/shenandoah/missing_evac_oom_scope/webrev.00/index.html

Test:

   tier3_gc_shenandoah (fastdebug and release)

Thanks,

-Zhengyu