mod_storage_sql: Compose a keyval+ store out of keyval and map store methods

Removes the need for the shim in storagemanager.

The methods only really access the 'store' property of the first (self)
argument, so this is safe.

plugins/mod_storage_sql.lua

@@ -660,10 +660,27 @@ function archive_store:users()
 	return iterator(result);
 end
 
+local keyvalplus = {
+	__index = {
+		-- keyval
+		get = keyval_store.get;
+		set = keyval_store.set;
+		items = keyval_store.users;
+		-- map
+		get_key = map_store.get;
+		set_key = map_store.set;
+		remove = map_store.remove;
+		set_keys = map_store.set_keys;
+		get_key_from_all = map_store.get_all;
+		delete_key_from_all = map_store.delete_all;
+	};
+}
+
 local stores = {
 	keyval = keyval_store;
 	map = map_store;
 	archive = archive_store;
+	["keyval+"] = keyvalplus;
 };
 
 --- Implement storage driver API