do_verification_job: fix "never-reverify" and refactor/comment

do_verification_job: fix "never-reverify" and refactor/comment

commit a4915dfc2bc7bef03354f97f5bbce9fe2df4e0d6 made a wrong fix, as
it did not observed that the last expressions was done under the
invariant that we had a last verification result, because if none
could be loaded we already returned true (include).

It thus broke the case for "never re-verify", which is important when
using multiple schedules, a more high frequent one for new,
unverified snapshots, and a low frequency to re-verify older snapshots,
e.g., monthly.

Fix this case again, rework the code to avoid this easy to oversee
invariant. Use a nested match to better express the implication of
each setting, and add some comments.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>