Discussion: Decouple cancel/drop mview from barrier

[kwannoel]

Support cancel of stream job without barrier.

• Usually when high barrier latency, some stream job can be stuck, but due to high barrier latency, it cannot be cancelled either.
• Another case: When high barrier latency for first barrier, stream job cannot be created, but cannot be cancelled either, until first barrier pass. If we can decouple cancel from barrier, we can avoid this.

Just noting this down first, didn't think of any ideas on how to support it yet.