Shenandoah: reconsider pacing updates memory ordering

Shenandoah pacer uses atomic operations to update budget, progress, allocations seen. 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 pacing, as we do not piggyback memory effects on it. All pacing updates can use "relaxed".