Support for scheduled work queue entry processing

If one wants to process a work queue entry at certain time, for example 24hrs after being put into the queue, one has to employ scheduler, which will periodically check if it is already time time to process the entry. This works, but it is misusing scheduler, whose main function is to perform tasks at given time.

We would like to introduce new feature that will allow us to specify time at which the work queue entry should be processed without need for scheduler.