Split off event processing pipelines.

[rfoxkendo]

• Rank 0 - MPI pump to queue up requests for data from rank 2+
• Rank 2+ are the event processors.
• Messaging format to send events.
• Event processors send event lists to rank 1 (histogramer).
• Rank 0 still reads data.
  • When it gets data it adds it to a fixed sized queue of work items. If that gets full it stops reading data (but this is not a dead-lock).
  • When a work items gets to the queue:
    • If the work item request queue is not empty the work item is forwarded to the top requestor in the queue.
    • If the request queue is empty the item is queued.
    • When a work item request is received, it is satsified from the work item queue if not empty or later when a work item is available.