RFR: [8u] Critical backports to sh/jdk8u

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/backports/jdk8u-20180126-crit/webrev.01/

We do not have much time to complete bulk backports, so let us backport only the critical
bug/perf/test fixes:

dc779781dd5e: Do not put down update-refs-in-progress flag concurrently
1d1238a0603b: Defer cleaning of system dictionary and friends to parallel cleaning phase
1a6a9f288dd2: Bitmap size might not be page aligned when large page is used
30e8ba6e2794: VerifyJCStressTest should test all heuristics
820129a799b1: Allocation failure injection machinery

Let's do these right now. We shall backport other things as time allows.

Testing: hotspot_gc_shenandoah {fastdebug|release}

Thanks,
-Aleksey