RFR: Make heap counters update completely asynchronous

[Roman Kennke]

Am 21.09.2017 um 18:40 schrieb Aleksey Shipilev:
> http://cr.openjdk.java.net/~shade/shenandoah/concthread-periodic/webrev.01/
>
> This uses the periodic tasks facility provided by VM to update the counters asynchronously.
> Allocation and GC paths are triggering the update, but do not perform the update themselves. This
> allows to simplify allocation path, capture GCLAB allocations without risking deadlocks, etc.
>
> Testing: hotspot_gc_shenandoah, allocation benchmarks
>
> Thanks,
> -Aleksey
>
I like it.

Do we really want/need to remove stuff from metaspaceCounters.cpp|hpp ?

Roman