idmap_ad: add and use ldap_timeout and fix LDAP server failover

idmap_ad: add and use ldap_timeout and fix LDAP server failover

The key parts are:

1. If an LDAP search fails with the hardcoded fatal error, remove the
retry. That would only retry the query against the same server, taken
from the DCINFO cache key. Instead, force a DC rediscovery.

2. Set a default ldap_timeout and pass it to tldap_search(). This
avoids tldap_search() hanging forever on a stale TCP connection.

3. The LDAP server idmap_ad is using is not necessarily the same DC
we're using for RPC, so in case we learn about a dead DC, put it in
the negative-conn-cache.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=15844

Signed-off-by: Ralph Boehme <slow@samba.org>
Reviewed-by: Guenther Deschner <gd@samba.org>
(cherry picked from commit 4d69ec473b7be763399c9787eda8e659a1582184)