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

commit a4915dfc2b 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>

src/backup/verify.rs

@@ -442,7 +442,7 @@ pub fn verify_backup_group(
         if filter(&info) == false {
             task_log!(
                 worker,
-                "SKIPPED: verify {}:{} (already verified)",
+                "SKIPPED: verify {}:{} (recently verified)",
                 datastore.name(),
                 info.backup_dir,
             );