RFR (S) 8242641: Shenandoah: clear live data and update TAMS optimistically

[Aleksey Shipilev]

RFE:
  https://bugs.openjdk.java.net/browse/JDK-8242641

After CM-with-UR removal, we are open to clear liveness data and update TAMS
concurrently/optimistically. Shaves off a few microseconds on large number of regions.

Webrev:
  https://cr.openjdk.java.net/~shade/8242641/webrev.01/

Testing: hotspot_gc_shenandoah, eyeballing benchmarks

-- 
Thanks,
-Aleksey