RFR: [9] 2018-02-23, Bulk backports to sh/jdk9

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/backports/jdk9-20180223/webrev.01/

This backports relevant bug and diagnostics fixes to sh/jdk9:

[backport] Refactor allocation seqnum handling: split mutator/GC allocs, merge with other metadata
handling
[backport] Denser ShHeapRegion status line
[backport] Capture degenerated GC in hs_err status line
[backport] Record Shenandoah events in hs_err events section
[backport] Fixed code roots scanning that might be bypassed during degenerated cycle
[backport] Consistent liveness for humongous regions
[backport] RP closures should accept NULL referents

Testing: hotspot_gc_shenandoah {fastdebug|release}

Thanks,
-Aleksey