BUG/MINOR: resolvers: answser item list was randomly purged or errors

In case of SRV records, The answer item list was purged by the
error callback of the first requester which considers the error
could not be safely ignored. It makes this item list unavailable
for subsequent requesters even if they consider the error
could be ignored.

On A resolution or do_resolve action error, the answer items were
never trashed.

This patch re-work the error callbacks and the code to check the return code
If a callback return 1, we consider the error was ignored and
the answer item list must be kept. At the opposite, If all error callbacks
of all requesters of the same resolution returns 0 the list will be purged

This patch should be backported as far as 2.0.

src/action.c

@@ -157,6 +157,12 @@ int act_resolution_cb(struct resolv_requester *requester, struct dns_counters *c
 	return 0;
 }
 
+/*
+ * Do resolve error management callback
+ * returns:
+ *  0 if we can trash answser items.
+ *  1 when safely ignored and we must kept answer items
+ */
 int act_resolution_error_cb(struct resolv_requester *requester, int error_code)
 {
 	struct stream *stream;