don't set and read from topic hash in parallel (#6831)

this was causing a test to fail, although very rarely

src/topics/recent.js

@@ -250,9 +250,10 @@ module.exports = function (Topics) {
 	};
 
 	Topics.updateLastPostTime = function (tid, lastposttime, callback) {
-		async.parallel([
-			function (next) {
 		async.waterfall([
+			function (next) {
+				Topics.setTopicField(tid, 'lastposttime', lastposttime, next);
+			},
 			function (next) {
 				Topics.getTopicFields(tid, ['cid', 'deleted', 'pinned'], next);
 			},
@@ -270,11 +271,6 @@ module.exports = function (Topics) {
 				}
 				async.series(tasks, next);
 			},
-				], next);
-			},
-			function (next) {
-				Topics.setTopicField(tid, 'lastposttime', lastposttime, next);
-			},
 		], function (err) {
 			callback(err);
 		});