RFR: Adaptive CSet selection selects excessively when memory is tight

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/adaptive-cset-garbage/webrev.01/

There is a trouble in adaptive cset selection that causes it to grossly overshoot cset when memory
is tight. When that happens, and we need to clean up enough memory to match free threshold, we would
have we have the "min memory to free" guidance, and there we should count *garbage* that would be
reclaimed, not the live cset.

Example where it fails: say we have 100G heap, we need to free 10G, and there are 1000 regions that
are 10% live. Each region is then 100M, has 10M live data, current LDS is 10G. Current adaptive cset
selection would erroneously choose 10G / 10M = 1000 regions for the collection set, evacuating all
these 10G, the entire LDS (*argh*)! While we need only 10G / 90M = 111 regions to reclaim those 10G,
evacuating only ~1G of live data.

Testing: tier3_gc_shenandoah, specjbb

Thanks,
-Aleksey