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

[Aleksey Shipilev]

RFE:
  https://bugs.openjdk.java.net/browse/JDK-8243495

This is a follow up for the base work laid in JDK-8239786. We can now make more verbose concurrent
phases logging, at least when roots are concerned.

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

Sample piece of GC log:
  https://cr.openjdk.java.net/~shade/8243495/gc-log-sample.txt

Testing: hotspot_gc_shenandoah, eyeballing logs; more aggressive testing is running

-- 
Thanks,
-Aleksey