RFR: Print message when heuristics changes the setting ergonomically

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/heuristics-ergo-msg/webrev.01/

This makes all heuristics print their decisions about the flags they set ergonomically, e.g.:

[0.004s][info][gc] Heuristics ergonomically sets -XX:ShenandoahAllocationThreshold=10
[0.004s][info][gc] Heuristics ergonomically sets -XX:ShenandoahUncommitDelay=5000
[0.004s][info][gc] Heuristics ergonomically sets -XX:ShenandoahGuaranteedGCInterval=30000
[0.004s][info][gc] Heuristics ergonomically sets -XX:ShenandoahGarbageThreshold=20

Testing: hotspot_gc_shenandoah

Thanks,
-Aleksey