Rely on positions from notifier rather than manually advancing them

syncapi/storage/shared/syncserver.go

@@ -629,7 +629,6 @@ func (d *Database) addReceiptDeltaToResponse(
 			}
 			read.User[receipt.UserID] = eduAPI.ReceiptTS{TS: receipt.Timestamp}
 			content[receipt.EventID] = read
-			res.NextBatch.ReceiptPosition++
 		}
 		ev.Content, err = json.Marshal(content)
 		if err != nil {

syncapi/sync/requestpool.go

@@ -325,7 +325,6 @@ func (rp *RequestPool) currentSyncForUser(req syncRequest, latestPos types.Strea
 		// Add the updates into the sync response.
 		for _, event := range events {
 			res.ToDevice.Events = append(res.ToDevice.Events, event.SendToDeviceEvent)
-			res.NextBatch.SendToDevicePosition++
 		}
 	}