As described in CBSE-8828, for a replica DCP stream. If a previous item that uses XATTRs is to be deleted, however it is not resident - then we must perform a bgfetch - see
MB-36087. When the bgfetch is complete we replay PassiveStream::messageReceived. However this time it fails because the last seqno now equals the sequence number of the deletion and so it is discarded.
- only set the last seqno to the seqno of the message just received, once the message has been completed or
- If an EWOULD_BLOCK occurs reserve the setting of the last seqno so the message can be received again.