Have a "Skip processing" option...

[jessehouwing]

As requested by tfsaggregator/tfsaggregator#276

It looks like we may be able to do this as follows:

• Have an in-memory static list of work item id + revision after save.
• Have a method that checks whether to skip next processing if it hits the revision in the list
• Remove the id+revision after check
• Provide this as an interface with basic in-memory solution for single server processing.
• Allow community to submit other processing options like State Server, SQL backend, something through PR.

Worst case the work item will reprocess if the list is lost due to a process restart. This will waste some CPU, but should be fine.