RFR (S) 8222186: Shenandoah should not uncommit below minimum heap size

[Aleksey Shipilev]

RFE:
  https://bugs.openjdk.java.net/browse/JDK-8222186

Fix:
  http://cr.openjdk.java.net/~shade/8222186/webrev.01/

There is some confusion what -Xms really means. Current code assumes it only means "initial" heap
size, which allows it to uncommit the heap below -Xms. However, users expect -Xms to mean "minimum
heap size" to set the low watermark for heap shrinkage. We need to amend Shenandoah to avoid
uncommitting below that watermark.

Testing: hotspot_gc_shenandoah (includes new test cases), eyeballing gc logs

-- 
Thanks,
-Aleksey