dnsdist: support skip hashing AR section for caching

dnsdist: support skip hashing AR section for caching

Use case arise that two clients with different udp payload size,
a customized client uses 4096 while glibc resolver uses 1232.
User would like to share cache result for the same query name,
type and class in this case. The downstream servers does not use
ECS and would not return different answers upon other EDNS
options so this is to add an option to support such use case.