RFR: Stats should not record past-shutdown events

[Aleksey Shipilev]

The decision to exit VM may come abruptly during concurrent phase. In this case,
it makes little sense to record the performance data, because we will most
likely record the degenerated cycle.

Fix:
  http://cr.openjdk.java.net/~shade/shenandoah/stats-skip-shutdown/webrev.01/

Testing: hotspot_gc_shenandoah

Thanks,
-Aleksey