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

[Roman Kennke]

Great!!

Do we still need both SINGLE_THREADED *and* CONCURRENT any longer?

ShenandoahClassLoaderDataRoots<CONCURRENT, SINGLE_THREADED>

Other than that, looks very good.

Roman

> 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
>