RFR: Concurrent uncommit should be recorded as GC event + perf

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/uncommit-events/webrev.01/

This implements two easy improvements in heap uncommit: recording the currently hidden event
properly in statistics to observe the real time concurrent thread spend dealing with uncommits, and
simple lock interleaving with SpinPause, like we do in FreeSet::recycle_trash.

Testing: tier3_gc_shenandoah, ad-hoc benchmarks

Thanks,
-Aleksey