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.

ref #157

refcounting lead to several errors, for example when:
* ixfr started to write changesets
* scheduler started applying changes to zonefile
* ixfr finished writing changesets
>>> race for node flags <<<
* scheduler finished
==> result: corrupted node flags, bad CRC

closes #210

refs #181

Move (and sometimes add) #include <config.h> at the top of
all *.c files.

Remove all #include ["<]config.h[">] from headers files.

Change-Id: I1df07602d0240affd95e12fccd122558300d2145