core/dns: emit CONFIG_CHANGED signal even if caching

With systemd-resolved, NetworkManager considers /etc/resolv.conf unmanaged. This breaks hostname lookups in a subtle way: when a new connection comes online, NM will initiate the hostname lookup before propagating DNS updates to systemd-resolved, which of course will cause the request to fail. And because NM doesn't update /etc/resolv.conf, it doesn't emit a CONFIG_CHANGED signal which would've restarted the lookup.

Fix this by emitting a signal also when using a caching DNS plugin.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1933863