BUG/MINOR; resolvers: Ignore DNS resolution for expired SRV item

BUG/MINOR; resolvers: Ignore DNS resolution for expired SRV item

If no ADD item is found for a SRV item in a SRV response, a DNS resolution
is triggered. When it succeeds, we must be sure the SRV item is still
alive. Otherwise the DNS resolution must be ignored.

This patch depends on the commit "MINOR: resolvers: Move last_seen time of
an ADD into its corresponding SRV item". Both must be backported as far as
2.2.

(cherry picked from commit 49531e8471a1c1ae9e1c7a6c1aeeb00713024beb)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>