winbindd: introduce update_trusted_domains_dc()

winbindd: introduce update_trusted_domains_dc()

Reloading trusts should reload every aspect of
the trust and also remove deleted trusts from
the winbindd _domain_list.

But pending requests still continue.

With this commit it is required that
async state structures use struct winbindd_domain_ref
instead of raw struct winbindd_domain pointers,
in order to usage of stale pointers.

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Ralph Boehme <slow@samba.org>