winbind: Use one queue for all domain children

winbind: Use one queue for all domain children

If we have multiple domain children, it's important
that the first idle child takes over the next waiting request.

Before we had the problem that a request could get stuck in the
queue of a busy child, while later requests could get served fine by
other children.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=13292

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Volker Lendecke <vl@samba.org>
Autobuild-User(master): Stefan Metzmacher <metze@samba.org>
Autobuild-Date(master): Fri Feb 23 09:04:23 CET 2018 on sn-devel-144