[syslog-ng] diskqueue framework patch

Hi all, due to the need for disk queue usage in PE 4.1 we had to extend the syslog-ng sources, in order to be able write plugins that implement this feature. This patch contains these modifications. Basically it extends the 'class' hierarchy (and 'virtual' functions, see the new LogDestDriver) so that plugins can define their own queue mechanisms (as some do this in PE modules) and these special queues can be assigned to destinations. -Attila NAGY (syslog-ng-dev)