RFR: [8u] 2018-02-27: Bulk backports to sh/jdk8u

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/backports/jdk8u-20180227/webrev.01/

This backports outstanding work from past few weeks to sh/jdk8u:

abcc68d17515: [backport] ShenandoahWriteBarrierNode::find_bottom_mem() fix
927e60a90261: [backport] Move barriers into typeArrayOop.hpp direct memory accessors
33fec4c5103f: [backport] Roots verification should take the special roots first
0fdb1403c046: [backport] Refactor allocation metadata handling
d01efdbba161: [backport] Denser ShHeapRegion status line
f17cd086a664: [backport] Record Shenandoah events in hs_err events section
0c83f38776f5: [backport] Fixed code roots scanning that might be bypassed during degenerated cycle
af376fee7138: [backport] Consistent liveness for humongous regions
521d2597f753: [backport] RP closures should accept NULL referents
2a2fe8eff7cc: [backport] ReferenceProcessor is_alive setup is racy
aea30526d0a4: [backport] Speed up asserts and verification, improve fastdebug builds performance

It still misses Degenerated GC, because it still fails the tests.

Testing: hotspot_gc_shenandoah {fastdebug|release}

Thanks,
-Aleksey