Long pause for processing weakrefs in DaCapo benchmark

[raell at web.de]

Hi Aleksey,
 
thank you very much for your help. By using your suggested adjustments 
ParallelGCThreads=6 & ShenandoahRefProcFrequency=1 the max pause 
reduced for about 50% and runtime improved by 20%. 
 
Best regards 
Ralph