RFR: 2018-03-19, Bulk backports to sh/jdk9

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/backports/jdk9-20180319/webrev.01/

This backports all recent work to sh/jdk9:

5789a22b66bd: [backport] Fix || and && chaining warnings in memnode.cpp
79d0e462ff4a: [backport] Missing OOMScope in ShenandoahFixRootsTask
a248f5651744: [backport] fix for alias analysis with ShenandoahBarriersForConst
dcc1ef08dc0e: [backport] FinalEvac pause to turn off evacuation
4366aab742d0: [backport] Little cleanup
7edfd55938a2: [backport] Tax-and-Spend allocation pacing
06ccceb9e78c: [backport] Rewire control loop to avoid double cleanup work
a295f6a1aa2f: [backport] Excessive asserts in marked_object_iterate
1c7331519023: [backport] Added missing header file for non-PCH build
685f5911964e: [backport] Remove unnecessary write-barrier
a1665e6feb5d: [backport] Provide non-taxable allocation slack at the beginning of the cycle

Testing: hotspot_gc_shenandoah {fastdebug|release}

Thanks,
-Aleksey