ts/udpsink: move sync on buffer to try_next

By moving sync on buffer ts to try_next, the resulting delay can be cancelled when a state transition occurs.

To prevent item loss, this requires first peeking the incoming item from the channel without popping it. After the delay has elasped, we can pop the item as the last await point in try_next: either it will be cancelled before popping or the popped item will be passed on to handle_item.

Also add flush which was missing from stop and flush_start transition actions.