lib/zonecut: avoid one kind of NS dependency cycles

lib/zonecut: avoid one kind of NS dependency cycles

The problem here was that we need to know which addresses are timed-out
(and not to be re-probed) much earlier than we do NS selection ATM -
that's because under some circumstances it affects the depth of NS
zone cut that we choose, i.e. if all addresses in a certain zone cut are
"bad" in a certain sense, we need to use a zone cut closer to the root,
because otherwise we'd get into a dependency cycle.