Shenandoah: reconsider heap statistics memory ordering

ShenandoahHeap collects heap-wide statistics (used, committed, etc). It does so by atomically updating them with default CASes. Unfortunately, Hotspot's default for atomic operations is memory_order_conservative, which emits two-way memory fences around the CASes at least on AArch64 and PPC64.

This is excessive for statistics gathering, and "relaxed" should be jsut as good.