- zonecut map is now truly a volatile cache to accelerate zone cut
retrieval, RDATA is stored in cache
- cache update is commited after each successfuly resolved query
- SBELT contains a static data as a fallback if the root server cannot
be found in the zone cut map, or the data is missing in the cache

this is a little hacked, I'd like to do list of nested
transactions later and short-lived read transactions.

TODO:
pruning, better removal, periodic cleanup, configuration

Slow start evaluates all NS in the preference list, using mean as
a sorting function. Invalidated NSs are pushed to the back of the
preference list.

This is incorrect, missing names should be resolved.