RFR: Rename "cancel_concgc" to "cancel_gc"

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/rename-cancel/webrev.01/

It always irks me to see "Cancel concurrent GC" from Degenerated -> Full GC upgrade.

Testing: hotspot_gc_shenandoah, eyeballing -Xlog:gc

Thanks,
-Aleksey