knot: don't fail if TTL mismatch, unify TTL processing

src/knot/ctl/commands.c

src/knot/nameserver/update.c

src/knot/updates/apply.c

src/knot/updates/changesets.c

src/knot/updates/ddns.c

src/knot/updates/zone-update.c

src/knot/zone/node.c

src/knot/zone/node.h

src/knot/zone/zonefile.c

tests-extra/tests/ddns/ttl/test.py

tests-extra/tests/zone/ttl-slave/data/ttl-mismatch.zone.1

tests-extra/tests/zone/ttl-slave/test.py

tests-extra/tests/zone/ttl_mismatch/data/ttl-mismatch.zone

tests-extra/tests/zone/ttl_mismatch/test.py