RFR [8u], 2018-04-19, Bulk backports to sh/jdk8u

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/backports/jdk8u-20180419/webrev.01/

This backports recent work to sh/jdk8u:

[backport] Only report GC pause time to GC MXBean + Re-fix memory managers and memory pools usage
and pause reporting
[backport] Add test to verify Shenandoah is not enabled by default, and enabled with the flag
[backport] Basic support for x86_32: build and run in STW configuration
[backport] Bitmap based ShHeapRegionSet

Please read through the patches, because backports were not trivial.

Testing: hotspot_gc_shenandoah {fastdebug|release}

Thanks,
-Aleksey