src/utils_cache.c: Fix incorrect checking of persistent thresholds.

src/utils_cache.c: Fix incorrect checking of persistent thresholds.

I attached a patch to solve a problem related with notifications. When a value
is missing for a while (2 intervals), a FAILURE notification is raised, and if
Persist is false, the notification is repeated each interval, but man page
says:

  If set to false (the default) then a notification is only generated if a
  value is out of range but the previous value was okay.

So, I think that is a bug, if not I'm sorry for the noise :)

Regards,
  Andres

P.S.: The problem is only a mixing in if conditions into uc_check_timeout
      function.

Signed-off-by: Florian Forster <octo@huhu.verplant.org>