Fire a 'Room.timelineReset' event when we get a gappy sync

We need to reset things at the UI level when we get a gappy sync, so give the
clients something to listen for.

Also add a bunch of tests for that bit of code.

lib/sync.js

@@ -84,7 +84,8 @@ SyncApi.prototype.createRoom = function(roomId) {
         timelineSupport: client.timelineSupport,
     });
     reEmit(client, room, ["Room.name", "Room.timeline", "Room.redaction",
-                          "Room.receipt", "Room.tags"]);
+                          "Room.receipt", "Room.tags",
+                          "Room.timelineReset"]);
     this._registerStateListeners(room);
     return room;
 };