BUG/MEDIUM: resolvers: Don't set an address-less server as UP

BUG/MEDIUM: resolvers: Don't set an address-less server as UP

when a server status is updated based on a SRV item, it is always set to UP,
regardless it has an IP address defined or not. For instance, if only a SRV
item is received, with no additional record, only the server hostname is
defined. We must wait to have an IP address to set the server as UP.

This patch must be backported as far as 2.2.

(cherry picked from commit ab177ac1f31b5c6c2e4f7d023923dd668cce3076)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>