RFR: Use peak occupancy to optimize/pessimize free_threshold in adaptive heuristics

[Roman Kennke]

Hi,

Aleksey suggested this: currently we're optimizing the free-threshold
after N successful GC cycles, and pessimize it whenever we run into OOM
(i.e. 100% heap occupancy), thus leading us into relatively frequent
degenerated marking or update-refs cycles. The idea is to pessimize
earlier. That means, observe the peak occupancy (right before
reclamation) and pessimize if peak occupancy is less than a threshold.
Here I'm re-using ShenandoahMinFreeThreshold for this. This should
balance our GC cycles around that threshold and leave some room for
alloc spikes, thus leading to fewer degen cycles or full-gcs.

http://cr.openjdk.java.net/~rkennke/adaptive-minfree/webrev.00/
<http://cr.openjdk.java.net/%7Erkennke/adaptive-minfree/webrev.00/>

Testing: hotspot_gc_shenandoah, specjvm-compiler

Ok?