Merge branch 'keep_deleted_keys' into 'master'

doc/man/knot.conf.5in

doc/reference.rst

src/knot/conf/schema.c

src/knot/conf/schema.h

src/knot/dnssec/context.c

src/knot/dnssec/kasp/policy.h

src/knot/dnssec/key-events.c

src/knot/dnssec/zone-keys.c

tests-extra/tests/dnssec/key_rollovers/test.py

tests-extra/tools/dnstest/server.py