RFR: Do not abandon RP discovery on conc cancel

[Aleksey Shipilev]

Hi,

I think our code that handles concurrent GC cancellation is still under belief
that cancellation means Full GC. Which makes it correct to abandon reference
discovery on cancel. But this is not true anymore for degenerate CM/UR -- we
*want* to continue with RP, and we don't want to lose references. It smells like
a bug.

The fix is to move abandonment to Full GC:
  http://cr.openjdk.java.net/~shade/shenandoah/fullgc-abandon-rp/webrev.01/

Testing: hotspot_gc_shenandoah

Thanks,
-Aleksey