OKAY notification in missing values broken.

Hello.

This is a slightly different version of https://github.com/collectd/collectd/pull/225

There are two changes form the original patch of @mce35:

Change the state to ``STATE_MISSING`` in ``ut_missing``
to ``uc_check_timeout`` after call ``plugin_dispatch_missing`` because
threshold is a plugin (maybe you can use another plugin to track missings...)

Also in ``uc_check_timeout`` instead of not call ``plugin_dispatch_missing`` 
if value is missing, not put in the value in the list of entries to be flushed if
it's missing.

I was using this feature in collectd 4, would be nice to recover it in collectd 5.

I've been using this patch for a couple of months in production.

In file ``treshold.c``, the function ``ut_check_threshold`` (registered with ``plugin_register_write``) call ``uc_get_rate`` for each value. 
If ``uc_get_rate`` return ``NULL`` for a missing value we never recive the ``OKAY``. 
 
``uc_get_rate`` should return ``NULL`` when the value is missing?
I think so.

then, I have a problem...