upgrade topics:tid to sorted set, topics:recent doesnt have deleted topics

src/database/mongo.js

@@ -684,6 +684,12 @@
 		});
 	}
 
+	module.isSortedSetMember = function(key, value, callback) {
+		module.sortedSetScore(key, value, function(err, score) {
+			callback(err, !!score);
+		});
+	}
+
 	module.sortedSetsScore = function(keys, value, callback) {
 		if(value !== null && value !== undefined) {
 			value = value.toString();