RFR (S) 8223759: Shenandoah should allow arbitrarily low initial heap size

[Aleksey Shipilev]

RFE:
  https://bugs.openjdk.java.net/browse/JDK-8223759

Shenandoah heap regions checks verify that initial heap size covers at least 10 regions. This means
the lowest uncommitted size is at least 10*RegionSize, which is a lot for large heaps. This seems to
be the reminiscent of old resizing scheme, where Shenandoah could have run with "initial heap" for a
long time, before GC code decided to resize. There is little sense it in now: Shenandoah would
commit regions up to max_capacity if needed, including during the initial barrage of allocations. We
should instead check that "max" heap size covers at least 10 regions.

Patch:
  http://cr.openjdk.java.net/~shade/8223759/webrev.01/

Testing: hotspot_gc_shenandoah

-- 
Thanks,
-Aleksey