Enable SLA for queue items

[GautamPoddar]

Adding optional capability of inbuilt prioritization driven by SLA (in minutes) in combination of existing Created timestamp and Priority logic to select queue items (Get Next Item) will allow to meet any SLA (if applicable) for different types of work items within same Work Queue. This can be achieved with following approach - 
1. Create SLA (like Priority) field at Work Queue table.
2. Allow defining SLA value (like Priority) for 'Add To Queue' action.
3. The selection logic is modified to include SLA value also (in combination of 'Created' timestamp & 'Priority'). Alternative approach could be to add SLA value to 'Created' timestamp and populate 'Next Review' field which will allow the work item to picked accordingly.