+ changed RRSet comparing function to compare also TTLs

fixes #1030 @ 45m

Refs #1068 @3h

refs #643 @10m

- Fixed rehashing based on unittest output.
  TODO: there is still some condition on which it crashes on
        segfault; hard to replicate.
- Small fixes in zone, zone tests.

refs #643 @30m

refs #641 @1h30m

Added setting of previous node field. Removed assert that should not be there and was coushing abort with cz. zone (I removed that one before, too :D)
Closes #1069

- They were causing the compiler to crash.

- Zonedb always uses zone->name as the key.
- Removed assert from dnslib_node-parent(), replaced by check.
  (Probably should do the same with other such asserts there.)
- Reenabled adjust_rrsets() function (don't know why it was
  disabled).
- Fixed wrong assert in zone-contents

* Fixed XFR client termination
* Updated to new APIs

Conflicts:
	src/dnslib/zone.c
	src/knot/server/dthreads.c
	src/knot/server/xfr-handler.c
	src/knot/server/xfr-handler.h

- When the search in AVL tree went to the leftmost node, it did
  not return any 'previous' node. This was fixed in the AVL
  implementation by distinguishing this case. It may have impact
  on the performance as there is now one more condition jump
  for each encountered node during search. But it seems there is
  no better way around it.
- Fixed the usages of the tree-finding functions.

fixes #1065 @2h

Commit refs #1063.

- Setting zone pointer to NSEC3 nodes inserted into the zone.
- Fixed searching for NSEC3 node for some name
  (find_nsec3_for_name()) - there was a typo (name instead of
   nsec3_name, and not freeing the nsec3_name resulted in leak).
- Small fixes in debug output.

refs #1056 @5m

refs #1056 @5m

refs #1056 @25m

- Some usages still remain, they would require more systematic
  changes.

refs #1056 @2h

closes #1059 @10m

refs #1058 @15m

- Modified dname-table API a bit.
- Just switching old node pointer to new node pointer.
- Should not generate any problems.

closes #1058 @30m

refs #1046 @10m

- Setting the ptr in zone_add_node() function.
TODO: will have to be done other way when zone-contents instead
      of zone is used.

refs #1046 @20m

refs #1038 @15m, #1039 @15m

TODO: change the use of the API.
Note: Will not compile now

refs #1038 @20m @1039 @20m

- marking old node as old

- Separate functions for adding normal RRSets and RRSIG RRSets.
- Small fixes.

TODO: Ensure that all data are properly deallocated when the update
      fails!!

refs #1042 @45m

- Separated handling of RRSIGs from handling of normal RRSets.
- If RRSIGs are removed from RRSet, and the RRSet is already
  clear, the RRSet is removed as well.

refs #1041 @1h