shaping.lua: increase shaping_data cache capacity

I've observed a system where we had more than 4 outstanding
shaping_data lookups from different ConfigEpochs; that system
had slow dns resolution and we ended up looping for these because
the capacity was not high enough and the population of the result
always took too long.

Let's just give ourselves a bit more headroom; this coupled
with the earlier commits that adjust retry behavior should
help to smooth things out in this situation.

assets/policy-extras/shaping.lua

@@ -413,7 +413,7 @@ function mod:setup_with_automation(options)
   local cached_load_data = kumo.memoize(load_shaping_data, {
     name = 'shaping_data',
     ttl = options.cache_ttl or '1 minute',
-    capacity = 4,
+    capacity = 16,
     invalidate_with_epoch = true,
     retry_on_populate_timeout = options.retry_shaping_data_fetch_on_populate_timeout,
     populate_timeout = options.shaping_data_populate_timeout,