[11u] RFR/RFC: Bulk backport to sh/jdk11

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/backports/jdk11-20190516/webrev.01/

This bunch includes:

 8223980: Shenandoah: Refactor and fix ObjArrayChunkedTask verification
 8223762: Shenandoah: overflows in calculations involving heap capacity
 8223759: Shenandoah should allow arbitrarily low initial heap size
 8223427: [TESTBUG] Disable JTReg Shenandoah tests when Graal is enabled
 8223447: Stabilize gc/shenandoah/TestStringDedupStress test
 8223446: Shenandoah breaks alignment with some HumongousThreshold values

Testing: hotspot_gc_shenandoah {fastdebug|release}, also with Graal enabled

-- 
Thanks,
-Aleksey