dnssec: small changes and fixes

- preparation for non-forced zone sing planning
- added a posibility not to wait for readers in changeset application when applying DNSSEC changes upon load/reload (there should be no readers, since the zone is not in the zonedb)
- fixes in changeset merging (SOAs and serials were wrong - SOAs might still be wrong, when me merge the changesets, we do *NOT* want to update the serial, since the user already did that)

Refs #4