Efficient node data storage, libknot refactoring

Changes:

- new file rr.h:
  - contains functions and structures that work with RRs or RR arrays.
  - this enables us to have RRSet-like functionality inside `node_t`

- node no longer stores `knot_rrset_t`, but only structure containing type, data and additional nodes.

- RRSet API cleaned up a lot:
 - RRSet is now an entrypoint into zone structure
 - It is no longer possible to create RRSet that is not sorted or one that contains duplicates
 - Obsolete getter functions removed

- Node and zone contents API cleaned up

- DDNS rewritten to changeset creation only.
 - Still far from perfect, but this is the best I can do without new zone API
 - Over-complicated prereq check rewritten too

- Changeset application greatly simplified, no need to explicitly handle changes anymore
 - Application now preserves the changeset structure, everything is copied
  - We could probably simplify the code a lot because of this at many places