timer-db config support

Allow specification of persistent timer database location.

See merge request !461

found by LibFuzzer

backported from 2.0, not cricical in 1.6

$enable_daemon is not defined in 1.6.x and it was making the systemd integration to fail (fixes #432)

- 57746bb3  zone: add a flag to mark expired zone
- c3157d8a  timers: workaround to store that zone has expired
- 443a4609  zonedb: refuse to reload expired zone
- 25c24feb  zonedb: refuse to load expired zone
- 6e0a7396  zone loading: fix typo causing early expiration

If a node to be included into an NSEC proof was establishing
a delegation, it was incorrectly skipped in the lookup.

close #415

fix lost notification messages

See merge request !423

The scheduled time of an event must not be changed when the event is
potentially in the heap and the heap is not locked. Otherwise a parallel
modification of the heap could break the heap property and the planning
might not work reliably.

closes #390

zone: add request EDNS option support (LTS)

See merge request !413

Search for unified libsystemd first then for older libsystemd-journal and libsystemd-daemon (Initial patch by Cameron Norman)

Multi-master failover

closes #372

See merge request !386