RFR: [10] 2018-02-23, Bulk backports to sh/jdk10

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/backports/jdk10-20180223/webrev.01/

This is trivial, because sh/jdk and sh/jdk10 are not that far apart. All patches apply without any
conflicts.

[backport] Weaken assert in Traversal GC
[backport] Refactor allocation seqnum handling: split mutator/GC allocs, merge with other metadata
handling
[backport] Denser ShHeapRegion status line
[backport] Traversal: Don't traverse new objects
[backport] Make Traversal share code with conc-mark
[backport] Traversal: Initialize extra liveness arrays when conc-threads > par-threads
[backport] Temporarily disable ThreadLocalHandshakes with Shenandoah
[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