RFR(S): Use scoped object for gc session/phases recording

[Zhengyu Gu]

Using scoped object for recording gc session and gc phases, to avoid the 
awkward do ... while(false) loop


Webrev:http://cr.openjdk.java.net/~zgu/shenandoah/gc_phase_recording/webrev.00/

Thanks,

-Zhengyu