RFR: [10] 2018-03-05: Bulk backports to sh/jdk10

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/backports/jdk10-20180305/webrev.01/

a7a034e0dafc: [backport] Arraycopy fixes (tests and infrastructure)
1fd7115d90f4: [backport] Fixed pinned region handling in mark-compact
58d091ca55ba: [backport] Cleanup allocation tracking in heuristics
d9333bb79c2c: [backport] Setup process references and class unloading once before the cycle
fbbd713f1211: [backport] Print message when heuristics changes the setting ergonomically
6a2a2deffcab: [backport] Refactor uncommit handling: react on explicit GCs, feature kill flag, etc
3c0dbf1ed0fd: [backport] "Compact" heuristics for dense footprint scenarios

The only thing this backport batch is missing is the actual arraycopy-checkcast fix (we backport
tests and heap infrastructure, but not the actual BarrierSet changes), which requires a more careful
backporting. Roman, please take care of this?

Testing: hotspot_gc_shenandoah {fastdebug|release} (except new failing test)

Thanks,
-Aleksey