RFR (M) 8239786: Shenandoah: print per-cycle statistics

[Roman Kennke]

Do I see it correctly that we get this with -Xlog:gc+stats=info ? I.e.
the same that gives us the end-of-run-stats? Might that be too spammy
for info? Maybe wire it to 'trace' instead?

The output is absolutely nice and very useful! Well done!

Roman

> RFE:
>   https://bugs.openjdk.java.net/browse/JDK-8239786
> 
> A brief tour:
>  *) ShPhaseTimings now collects worker data and cycle data into the separate counters that are
> populated once per cycle. ShControlThread is then responsible for merging and printing out that data.
>  *) We still have "global" view that is printed at the end of the run, but we also have "cycle" view
> that is printed after each cycle. Note that printing is outside of any pause, and so the logging is
> effectively asynchronous!
>  *) Where possible, "cycle" view shows the per-worker statistics.
> 
> Webrev:
>   https://cr.openjdk.java.net/~shade/8239786/webrev.01/
> 
> Sample piece of GC log:
>   https://cr.openjdk.java.net/~shade/8239786/gc-log-sample.txt
> 
> Testing: hotspot_gc_shenandoah, eyeballing logs; more aggressive testing is running
>