doc: add warning about timers infuencing zsk key lifetime

closes issue #481

See merge request !603

zonedb: fix typo

http://boostrap.com/ ;-)

Waiting for https://gitlab.labs.nic.cz/labs/knot/merge_requests/587

See merge request !598

Don't preserve CD flag

See merge request !602

internet: skip empty non-terminal parents during delegation lookup

See merge request !600

http://boostrap.com/ ;-)

QP coverity

See merge request !596

I think the reported "issues" couldn't really happen,
but it seems better to refactor to get rid of them.

Zone refresh fixes

fixes #479

See merge request !594

eliminated compile warnings: deprecated readdir_r -> readdir; Wuninitialized few variables on make check

QP trie integration

See !574. See original merge request !593 for more details.