No checkpoint items are to be added to a stream's readyQ if the stream was forcefully closed

After a stream has been marked as dead, a streamEnd message is added into the readyQ, after this point no other mutation/deletion/marker is to be added into the readyQ.

A race between closeStream/closeSlowStream and the active stream's nextCheckpointTask could cause this scenario, where mutations/deletions and a marker can get added to the readyQ for the stream, after a STREAM_END has been queued.

This is to avoid shipping unnecessary items after a stop request has been issued for a stream.