MINOR: resolvers: Purge answer items when a SRV resolution triggers an error

MINOR: resolvers: Purge answer items when a SRV resolution triggers an error

When a SRV request trigger an error, if we decide to handle the error
because last_valid duration is expired, the answer list may be purged. All
items are considered as obsolete.

(cherry picked from commit 51d5e3bda7fb07a5f03cb878dd66f673c3ed1a59)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
(cherry picked from commit 720ed09b491f5005741115bd36855e05deb99481)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
(cherry picked from commit f48a118811b27ee5dbdbe80603cc0fc7e9e4dbb3)
[cf: Must be backported as far as 2.0 because of recent changes on resolvers]
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>