[8] 2020-05-08, Bulk backports to sh/jdk8

[Aleksey Shipilev]

https://cr.openjdk.java.net/~shade/shenandoah/backports/jdk8u-20200508/webrev.01/

Here is a bunch of backports for 8u:

 [backport] 8242353: Shenandoah: micro-optimize region liveness handling
 [backport] 8242365: Shenandoah: use uint16_t instead of jushort for liveness cache
 [backport] 8241142: Shenandoah: should not use parallel reference processing with single GC thread
 [backport] 8242641: Shenandoah: clear live data and update TAMS optimistically
 [backport] 8243238: Shenandoah: explicit GC request should wait for a complete GC cycle
 [backport] 8243307: Shenandoah: remove ShCollectionSet::live_data
 [backport] 8243301: Shenandoah: ditch ShenandoahAllowMixedAllocs
 [backport] 8243395: Shenandoah: demote guarantee in ShenandoahPhaseTimings::record_workers_end
 [backport] 8244551: Shenandoah: Fix racy update of update_watermark
 [backport] 8243463: Shenandoah: ditch total_pause counters
 [backport] 8243464: Shenandoah: print statistic counters in time order
 [backport] 8243465: Shenandoah: ditch unused pause_other, conc_other counters
 [backport] 8243487: Shenandoah: make _num_phases illegal phase type
 [backport] 8244200: Shenandoah: build breakages after JDK-8241743
 [backport] 8242229: Shenandoah: inline ShenandoahHeapRegion liveness-related methods

Testing: hotspot_gc_shenandoah {fastdebug,release}

-- 
Thanks,
-Aleksey