RFR: 2018-03-19 (2), Bulk backports to sh/jdk10

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/backports/jdk10-20180319-2/webrev.01/

We have done a few trivial fixes today, and those fixes do not need to wait another backport cycle.
Current sh/jdk should pass the first round of nightly testing in a few hours.

094fd848894c: [backport] Remove runtime storeval barrier
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