RFR (XS): Cancel events should print cause, and fire only once

[Aleksey Shipilev]

Hi,

This is simple UX improvement that makes sure we only print "Cancel GC"
once, and also print the reason for it:
  http://cr.openjdk.java.net/~shade/shenandoah/cancel-cause/webrev.01/

Thanks,
-Aleksey