libknot/packet: compression algorithm improved

The previous compression algorithm does not compress everything it can
in cases where there are domain names in the data section of a RR, e.g.
when the answer contains

bar.example.com  CNAME  foo.example.com
foo.example.com  A      192.0.0.2

and foo.example.com was not encountered previously, then the second RR
leaves the 'foo' part uncompressed.

The improvement resolves some of these cases by comparing the owner of a new RRset to be written
with the last written dname instead of QNAME. Also, the effectivity was slightly increased
according to benchmarks.