MEDIUM: dns: use Additional records from SRV responses

MEDIUM: dns: use Additional records from SRV responses

Most DNS servers provide A/AAAA records in the Additional section of a
response, which correspond to the SRV records from the Answer section:

  ;; QUESTION SECTION:
  ;_http._tcp.be1.domain.tld.     IN      SRV

  ;; ANSWER SECTION:
  _http._tcp.be1.domain.tld. 3600 IN      SRV     5 500 80 A1.domain.tld.
  _http._tcp.be1.domain.tld. 3600 IN      SRV     5 500 80 A8.domain.tld.
  _http._tcp.be1.domain.tld. 3600 IN      SRV     5 500 80 A5.domain.tld.
  _http._tcp.be1.domain.tld. 3600 IN      SRV     5 500 80 A6.domain.tld.
  _http._tcp.be1.domain.tld. 3600 IN      SRV     5 500 80 A4.domain.tld.
  _http._tcp.be1.domain.tld. 3600 IN      SRV     5 500 80 A3.domain.tld.
  _http._tcp.be1.domain.tld. 3600 IN      SRV     5 500 80 A2.domain.tld.
  _http._tcp.be1.domain.tld. 3600 IN      SRV     5 500 80 A7.domain.tld.

  ;; ADDITIONAL SECTION:
  A1.domain.tld.          3600    IN      A       192.168.0.1
  A8.domain.tld.          3600    IN      A       192.168.0.8
  A5.domain.tld.          3600    IN      A       192.168.0.5
  A6.domain.tld.          3600    IN      A       192.168.0.6
  A4.domain.tld.          3600    IN      A       192.168.0.4
  A3.domain.tld.          3600    IN      A       192.168.0.3
  A2.domain.tld.          3600    IN      A       192.168.0.2
  A7.domain.tld.          3600    IN      A       192.168.0.7

SRV record support was introduced in HAProxy 1.8 and the first design
did not take into account the records from the Additional section.
Instead, a new resolution is associated to each server with its relevant
FQDN.
This behavior generates a lot of DNS requests (1 SRV + 1 per server
associated).

This patch aims at fixing this by:
- when a DNS response is validated, we associate A/AAAA records to
  relevant SRV ones
- set a flag on associated servers to prevent them from running a DNS
  resolution for said FADN
- update server IP address with information found in the Additional
  section

If no relevant record can be found in the Additional section, then
HAProxy will failback to running a dedicated resolution for this server,
as it used to do.
This behavior is the one described in RFC 2782.