Ttl Tests

Ttl Check Fix

More info in the commit message.

Rollback Fixes

Some rollback calls were missing, I've added them, but I had to hack it a bit, there's a comment with explanation.

Previous logic was too complicated and confusing. Currently, there are
just two pregenerated source files which are selected on the basis of
the --disable-fastparser configure option.

Pregenerated files can be updated using scripts/update-parser.sh script.

- in DDNS, the check would not work if the node was previously somehow
  changed, the check in changeset application should take care of this.

- check in changeset application would not work because it was triggered
  only when adding RRs into empty node.

Use knot_rdata_t directly instead of via knot_rrset_t

Hostname acquisition

Coverity #1197142

getter purge

Some more unittests