Fixed usage of task_arena from tbb in HLTControlFlowMgr

This had been broken in the past for Allen, in order to work around the lack of nextEvent in the IQueueingEventProcessor interface. Expect this was a pure mistake, nextEvent has always been there. Allen now uses it and thus we can go back to proper code in HLTControlFlowMgr

Goes together with Allen!1592 (merged)