Stats Lock Free Non Final Ref Count Releases

Reduced the cost of releasing references to stats: non-final reference-count decrements no longer take the allocator's lock (for counters, gauges and text readouts) or the store's histogram lock (for histograms), and stat reference-count operations use standard reference-counting memory orderings instead of sequentially-consistent ones. Processes with very large numbers of stats should see substantially lower and more consistent flush times (roughly 30-40% in the stats flush benchmark at millions of stats). There are no visible behavioral changes.