mod_pubsub,mod_pep: Implement 'send_last_published_item' option #1436

Default left as 'never' in mod_pubsub to preserve the previous behavior.
Unclear if this is desirable, but can always be changed later.

In mod_pep this allows turning off the automatic resending of most
recent item.

util/pubsub.lua

@@ -136,6 +136,7 @@ local default_node_config = {
 	["max_items"] = 20;
 	["access_model"] = "open";
 	["publish_model"] = "publishers";
+	["send_last_published_item"] = "never";
 };
 local default_node_config_mt = { __index = default_node_config };