RFR: Make heap counters update completely asynchronous

[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