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?
> 
> I would prefer to keep it gc+stats=info, for several reasons:
>  *) gc+stats is not enabled by default, and it is inherently opt-in;
>  *) this logging is asynchronous, so bulk printing does not come with performance overhead;
>  *) "trace" is usually for very fine-grained tracing, and this is rather high level.
>  *) I think ZGC prints out the similar bunch of counters under gc+stats as well;

Ok.

> This might change once we reshuffle logging further...
> 
> I updated webrev a bit (copyrights, excess headers):
>   https://cr.openjdk.java.net/~shade/8239786/webrev.02/
> 
> Ok?

Ok!

Thank you,
Roman