RFR: Added logging for the number of workers used for GC cycles

[Aleksey Shipilev]

On 06/11/2018 06:05 PM, Zhengyu Gu wrote:
>> Can't we put the loggers into ShenandoahWorkerScope constructor itself? I don't think we need "for
>> XXX" messages all that much, because we can relate that with the other log lines, this simplifies
>> the whole thing.
> I would argument that, if apply gc+task filter, then the messages will be out-of-context without
> "for XXX".
> 
> Does ShenandoahWorkerScope(workers, n_workers, for XXX), so move logging into constructor, work for
> you?

I think it makes our lives harder without much of the benefit. But yeah, that should trim down the
code noise:

    ShenandoahWorkerScope scope(workers(),
                                ShenandoahWorkerPolicy::calc_workers_for_conc_update_ref(),
                                "conc update refs");

Thanks,
-Aleksey