We got the digest available anyway, and it's only 16 bytes more to
save (compared to last_generation and the recently removed last_time,
both being 64 bit = 8 bytes each)
Side benefit, we detect config changes made manually (e.g., `vim
datacenter.cfg`) immediately.
Note that we could restructure the maintenance mode checking to only
be done after checking if there's a cached datastore, in which case
using the generation could make sense to decide if we need to re-load
it again before blindly loading the config anyway. As that's not only
some (not exactly hard but not really trivial like a typo fix either)
restructuring work but also means we'd lose the "detect manual
changes" again I'd rather keep using the digest.
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>
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