The general idea is a processor function which takes list_t of someting
and passes it to the specific processing function one by one.
AXFR uses list of two zone trees, IXFR list of changesets.

Also removed some unused code.

ref #121

ref #121

ref #121

ref #121

Since inserting into hashtable may fail, this would result in lost
records. I refactored the code a bit and hashtable node split is now
done recursively, i.e. it divides nodes until it fits.

Sorted iteration is not needed and hhash doesn't support sorted
iteration unless an order index is built.

Use pointer-based thread identifiers instead of number-based ones. This
should resolve compilation warnings on BSD systems.

Linux uses integers as thread identifiers, BSD based systems use pointers.

ref #157