RFR (S) 8247593: Shenandoah: should not block pacing reporters

[Aleksey Shipilev]

Bug:
  https://bugs.openjdk.java.net/browse/JDK-8247593

After JDK-8247358, we are acquiring the _wait_monitor in ShenandoahPacer::report_internal. That runs
into potential deadlocks with threads that are waiting on the same lock *and* safepointing at the
same time, against the concurrent workers that want to report the progress before returning for
subsequent safepoint.

Fix:
  https://cr.openjdk.java.net/~shade/8247593/webrev.01/

Testing: hotspot_gc_shenandoah (some tests timed out before); ad-hoc benchmarks

-- 
Thanks,
-Aleksey