Do not add request to a wait chain that's already processed or being processed.

Do not add request to a wait chain that's already processed or being processed.

The following scenario can occur. Multiple concurrent clients doing the same query A
are needed to trigger it:

1. Incoming request A, which has a need for request X
2. Add request X to chain because we already have an identical outstanding request
3. We receive the reply for X
4. We process the chain
5. In the meantime a new request for X that's identical is added to the chain
6. The added id in step 5 is not being processed anymore -> timeout

This can happen if request X has TTL 0, otherwise the record cache would have a hit.