MAJOR: server: add DNS-based server name resolution

Relies on the DNS protocol freshly implemented in HAProxy.
It performs a server IP addr resolution based on a server hostname.
diff --git a/include/proto/checks.h b/include/proto/checks.h
index 67d659f..9ab3e50 100644
--- a/include/proto/checks.h
+++ b/include/proto/checks.h
@@ -29,6 +29,7 @@
 const char *get_check_status_info(short check_status);
 int start_checks();
 void __health_adjust(struct server *s, short status);
+int trigger_resolution(struct server *s);
 
 extern struct data_cb check_conn_cb;