memory: use relaxed memory order for incrementing cross-thread stats
memory: use relaxed memory order for incrementing cross-thread stats

The default, sequentially consistent increment, is expensive on some
architectures. This isn't a big deal since this is a slow path, but
is still worth it.

cloudius-systems/seastar,syuu1228/seastar,avikivity/seastar,scylladb/seastar,syuu1228/seastar,syuu1228/seastar,scylladb/seastar,avikivity/seastar,scylladb/seastar,cloudius-systems/seastar,avikivity/seastar,cloudius-systems/seastar