BUG/MEDIUM: resolvers: Reset server address and port for obselete SRV records

When a SRV record expires, the ip/port assigned to the associated server are
now removed. Otherwise, the server is stopped but keeps its ip/port while
the server hostname is removed. It is confusing when the servers state are
retrieve on the CLI and may be a problem if saved in a server-state
file. Because the reload may fail because of this inconsistency.

Here is an example:

 * Declare a server template in a backend, using the resolver <dns>

server-template test 2 _http._tcp.example.com resolvers dns check

 * 2 SRV records are announced with the corresponding additional
   records. Thus, 2 servers are filled. Here is the "show servers state"
   output :

2 frt 1 test1 192.168.1.1 2 64 0 1 2 15 3 4 6 0 0 0 http1.example.com 8001 _http._tcp.example.com 0 0 - - 0
2 frt 2 test2 192.168.1.2 2 64 0 1 1 15 3 4 6 0 0 0 http2.example.com 8002 _http._tcp.example.com 0 0 - - 0

 * Then, one additional record is removed (or a SRV record is removed, the
   result is the same). Here is the new "show servers state" output :

2 frt 1 test1 192.168.1.1 2 64 0 1 38 15 3 4 6 0 0 0 http1.example.com 8001 _http._tcp.example.com 0 0 - - 0
2 frt 2 test2 192.168.1.2 0 96 0 1 19 15 3 0 14 0 0 0 - 8002 _http._tcp.example.com 0 0 - - 0

On reload, if a server-state file is used, this leads to undefined behaviors
depending on the configuration.

This patch should be backported as far as 2.0.

src/resolvers.c

@@ -577,6 +577,8 @@ static void resolv_check_response(struct resolv_resolution *res)
 						srv->hostname        = NULL;
 						srv->hostname_dn     = NULL;
 						srv->hostname_dn_len = 0;
+						memset(&srv->addr, 0, sizeof(srv->addr));
+						srv->svc_port = 0;
 						resolv_unlink_resolution(srv->resolv_requester);
 					}
 					HA_SPIN_UNLOCK(SERVER_LOCK, &srv->lock);

src/server.c

@@ -4135,6 +4135,8 @@ int srvrq_resolution_error_cb(struct resolv_requester *requester, int error_code
 			s->hostname        = NULL;
 			s->hostname_dn     = NULL;
 			s->hostname_dn_len = 0;
+			memset(&s->addr, 0, sizeof(s->addr));
+			s->svc_port = 0;
 			resolv_unlink_resolution(s->resolv_requester);
 		}
 		HA_SPIN_UNLOCK(SERVER_LOCK, &s->lock);