improved zonedb search, fixed root zone case + basic unit tests #242

HAT-trie zone database wasn't effective for small number of zones,
because hashing presented a constant penalty. New zone database
is represented by an array of zones, sorted by number of labels and
then lexicographically. Zones are then grouped by the label count
into stack, so obviously mismatching names are pruned when searching.
Each labelcount group is then search either linearly or using binary
search based on the array length.