delete method
Discards the stored cursor so the next subscription starts from the live edge.
This is not merely a convenience: a relay answers a cursor that is ahead
of its own stream with a FutureCursor error, which repeats on every
reconnect. Without a way to discard the cursor, a consumer that has read
past its relay — because the relay was rolled back, or because it was
pointed at a different relay — reconnects forever and never recovers.
Implementation
@override
Future<void> delete() async => _cursor = null;