MEDIUM: resolvers: add a ref on server to the used A/AAAA answer item

MEDIUM: resolvers: add a ref on server to the used A/AAAA answer item

This patch adds a head list into answer items on servers which use
this record to set their IPs. It makes lookup on duplicated ip faster and
allow to check immediatly if an item is still valid renewing the IP.

This results in better performances on A/AAAA resolutions.

This is an optimization but it could avoid to trigger the haproxy's
internal wathdog in some circumstances. And for this reason
it should be backported as far we can (2.0 ?)