[Room] Store prevBatch when requesting history

lib/src/Room.dart

@@ -300,6 +300,9 @@ class Room {
 
     if (resp is ErrorResponse) return;
 
+    prev_batch = resp["end"];
+    client.store?.storeRoomPrevBatch(this);
+
     if (!(resp["chunk"] is List<dynamic> &&
         resp["chunk"].length > 0 &&
         resp["end"] is String)) return;

lib/src/Store.dart

@@ -149,6 +149,12 @@ class Store {
     return null;
   }
 
+  Future<void> storeRoomPrevBatch(Room room) async {
+    await _db.rawUpdate(
+        "UPDATE Rooms SET prev_batch=? WHERE id=?", [room.prev_batch, room.id]);
+    return null;
+  }
+
   /// Stores a RoomUpdate object in the database. Must be called inside of
   /// [transaction].
   Future<void> storeRoomUpdate(RoomUpdate roomUpdate) {

test/Timeline_test.dart

@@ -198,6 +198,7 @@ void main() {
       expect(timeline.events[6].id, "1143273582443PhrSn:example.org");
       expect(timeline.events[7].id, "2143273582443PhrSn:example.org");
       expect(timeline.events[8].id, "3143273582443PhrSn:example.org");
+      expect(room.prev_batch, "t47409-4357353_219380_26003_2265");
     });
   });
 }