pool: Track decode blocks without parent lineage (from prefill)

[RishabhSaini]

Previously in PD Disagg mode, the decode KV Cache events were dropped since their parents were generated in the Prefill pods

[yankay]

I think this change is reasonable, but I have a follow-up question: after the key is properly added to the index, will the routing process perform a lookup and select the appropriate decoding node?

[vMaroon]

Can you verify that KVEvents are not generated after transferring KVs from prefill to decode? AFAIK the mechanism should be working. If it doesn't, then it's a bug in vLLM.

[RishabhSaini]

Looks like there is a race condition here when doing PD Disagg causing Parent block key not found in index:

Prefill Pod -> ZMQ PUB -> EPP SUB (goroutine 1) -> Worker Queue 0 -> digestEvents
Decode Pod -> ZMQ PUB -> EPP SUB (goroutine 2) -> Worker Queue 1 -> digestEvents

1. Prefill and decode publish events independently - no synchronization between pods
2. EPP uses separate ZMQ subscribers per pod - each runs in its own goroutine
3. Events from different pods go to different worker queues - no ordering guarantee across pods
4. Decode's events can be processed before prefill's - causing parent block lookup to fail