winbind: Improve child selection

winbind: Improve child selection

This improves the situation when a client request blocks a winbind
child. This might be a slow samlogon or lookupnames to a domain that's
far away. With random selection of the child for new request coming in
we could end up with a long queue when other, non-blocked children
could serve those new requests. Choose the shortest queue.

This is an immediate and simple fix. Step two will be to have a
per-domain and not a per-child queue. Right now we're pre-selecting
the check-out queue at Fry's randomly without looking at the queue
length. With this change we're picking the shortest queue. The better
change will be what Fry's really does: One central queue and red/green
lights on the busy/free checkout counters.

Signed-off-by: Volker Lendecke <vl@samba.org>
Reviewed-by: Andreas Schneider <asn@samba.org>
Autobuild-User(master): Volker Lendecke <vl@samba.org>
Autobuild-Date(master): Mon Feb 12 19:51:35 CET 2018 on sn-devel-144