CVE-2020-25717 winbindd: add generic wb_parent_idmap_setup_send/recv() helpers

CVE-2020-25717 winbindd: add generic wb_parent_idmap_setup_send/recv() helpers

This is more or less a copy of wb_xids2sids_init_dom_maps_send/recv,
but it's more generic and doesn't imply global state.

It also closes a initialization race by using a tevent_queue to
serialize the calls.

In the next commits we'll replace wb_xids2sids_init_dom_maps_send/recv.

We'll also use the new function in the wb_sids2xids code.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=14539

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Gary Lockyer <gary@catalyst.net.nz>
BUG: https://bugzilla.samba.org/show_bug.cgi?id=14556

(cherry picked from commit 209e81a2ea8c972ee57e2f0c9579da843c0e2ac7)