Drop the NZF (New Zone File) fallback for persisting runtime zone
configurations, making LMDB (NZD) the only storage backend. This
removes all #ifdef HAVE_LMDB conditionals, the meson 'lmdb' option,
and the NZF-related functions. LMDB is now a mandatory build
dependency.
The named-nzd2nzf tool is now always built.
Meson boolean options are usually configured with enabled/disabled
instead of on/off. Make things more consistent with other meson options
by renaming -Dnamed-lto=off to -Dnamed-lto=disabled.
After a couple releases, it appears that the minimum version can be
increased without much trouble.
Because meson only requires python 3.7 or greater, most supported
distributions can run a newer meson without having to deal with
additional dependencies or a new python version.
