Timeout for missing values

Timeout for missing values

Hi!

I attach here a patch which adds Timeout option in configuration file.
This option is global (i.e. at the same level as Interval) and it's
the max number of intervals that a package can be missed before a
missing notification would be raised. By dafault collectd wait 2
intervals and it's a hardcoded value AFAIK, if plugin cannot get data
for more than 2 intervals, then (if it's an interesting value),
collectd send a missing notification. Setting Timeout to an high value
you can be more tolerant with that missing.

It's usefull on large networks. In my case, the company LAN is
distributed on distant locations and sometimes (due to network issues)
UDP packages are lost, I use the Timeout to be more tolerant to this
networks fails. For example setting Internval to 10 and Timeout to 6,
a  missing notification will be raised only if none data was reported
in last 60s.

I hope that things explained well enough, and (who knows!) maybe this
could be usefull to anybody ;)

Regards,
  Andrés

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