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>