dnsdist: Detect ECS collisions in the packet cache

dnsdist: Detect ECS collisions in the packet cache

We did not detect a collision involving two queries for the same qname,
qtype, qclass and flags but with different ECS values hashing to the same key.
This commit adds an option to the packet cache to parse and keep the ECS value
in order to detect this kind of collisions.
It's not enabled by default because parsing the ECS value has a cost.