src/utils_cache.[ch]: Added the `uc_check_timeout' function.

src/utils_cache.[ch]: Added the `uc_check_timeout' function.

This function is called before the read plugins. It checks if values are
missing, i. e. have not been reported for longer than twice their ``interval''.
In this case a notification is created, though this is probably not the final
behavior.
This code is highly experimental.