MINOR: dns: implement a LRU cache for DNS resolutions

MINOR: dns: implement a LRU cache for DNS resolutions

Introduction of a DNS response LRU cache in HAProxy.

When a positive response is received from a DNS server, HAProxy stores
it in the struct resolution and then also populates a LRU cache with the
response.
For now, the key in the cache is a XXHASH64 of the hostname in the
domain name format concatened to the query type in string format.