Put speed holes in the code

We can make read receipts more efficient (and avoid double-animation) by using `PureComponent` which no-ops useless updates for us.

src/components/views/rooms/ReadReceiptMarker.js

@@ -32,7 +32,7 @@ try {
 } catch (e) {
 }
 
-export default class ReadReceiptMarker extends React.Component {
+export default class ReadReceiptMarker extends React.PureComponent {
     static propTypes = {
         // the RoomMember to show the RR for
         member: PropTypes.object,