[8u] RFR/RFC: Bulk backports to sh/jdk8

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/backports/jdk8u-20190517/webrev.01/

This bunch includes:

 [backport] 8223447: Stabilize gc/shenandoah/TestStringDedupStress test
 [backport] 8223446: Shenandoah breaks alignment with some HumongousThreshold values
 [backport] 8223759: Shenandoah should allow arbitrarily low initial heap size
 [backport] 8223762: Shenandoah: overflows in calculations involving heap capacity
 [backport] 8223980: Shenandoah: Refactor and fix ObjArrayChunkedTask verification
 [backport] 8223651: Build fails with --with-jvm-features=-jfr and --disable-precompiled-headers

It does require usual polishing for 8u: reshuffles, asserts with err_msgs, oopDesc::equals_unsafe,
missing align_*, etc.

Testing: Linux x86_64 hotspot_gc_shenandoah {fastdebug, release}

-- 
Thanks,
-Aleksey