s3:rpc_client: optimize the netlogon_creds_cli.tdb for read-only access

s3:rpc_client: optimize the netlogon_creds_cli.tdb for read-only access

Usually a record in this DB will be written once and then read
many times by winbindd processes on multiple nodes (when run in
a cluster). In order not to introduce a big performance penalty
with the increased correctness achieved by storing the netlogon
creds, in a cluster setup, we should activate ctdb's read only
record copies on this db.

Signed-off-by: Michael Adam <obnox@samba.org>
Reviewed-by: Stefan Metzmacher <metze@samba.org>