RFR(S): ShenandoahGCSession used wrong timer for full GC

[Zhengyu Gu]

ShenandoahGCSession should use STW timer instead of concurrent timer 
during full GC.

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

Test:

hotspot_gc_shenandoah: fastdebug and release

Thanks,

-Zhengyu