s3:winbind: Fork multiple children per domain

s3:winbind: Fork multiple children per domain

This makes us scale better with many simultaneous winbind requests,
some of which might be slow.

This implementation breaks offline logons, as the cached credentials are
maintained in a child (this needs fixing). So, if the offline logons are
active, only allow one DC connection.

Probably the offline logon and the scalable file server cases are
separate enough so that this patch is useful even with the restriction.