[14] RFR (S) 8243495: Shenandoah: print root statistics for concurrent weak/strong root phases

[Aleksey Shipilev]

Original RFE:
  https://bugs.openjdk.java.net/browse/JDK-8243495
  https://hg.openjdk.java.net/jdk/jdk/rev/45e615754576

This is a part of bulk backport of enhanced Shenandoah logging to 14u. Again, the context is a bit
different, because there is no weak/strong roots split. It is, however, good to have the logs for
"Concurrent Roots" phase too.

14u webrev:
  https://cr.openjdk.java.net/~shade/8243495/webrev.14u.01/

Testing: hotspot_gc_shenandoah, eyeballing gc logs

-- 
Thanks,
-Aleksey