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

Conflicts:
	src/knot/dnssec/zone-sign.c
	src/knot/updates/ddns.c
	src/knot/updates/xfr-in.c
	src/knot/zone/node.c
	src/knot/zone/node.h
	src/knot/zone/semantic-check.c
	src/knot/zone/zone-contents.c
	src/knot/zone/zone-contents.h
	src/knot/zone/zone-create.c
	src/libknot/rrset.c
	tests-extra/tests/ddns/basic/test.py

Note: ddns/basic fails now, but I don't think it's caused by my merge
jutsu.

- ns_put_rr function now accepts consts rrsets only
- fixed wrong TTL set in SOA copy
- removed a function move_rrset, to be replaced with RRSet function
  later.

Ddns When Zone Is Reloaded

See the commit message.

- all serialize/deserialize functions should be moved here eventually.

Update .travis.yml to not depend on build-dep command since Travis CI servers don't know knot package (yet)