HLTControlFlowMgr: create tbb::task_arena in start and check for existing arena

For the Allen geometry converters to support DD4HEP, they must make use of condition accessors and/or condition derivations. This requires all converters to become algorithms, as implemented in Allen!855 (merged). In production in the pit, the HLTControlFlowMgr does not run the event loop, but the converter algorithms still need to be called with valid conditions, and a small synthetic event loop containing only the converters is setup.

HLTControlFlowMgr manages its thread pool inside the nextEvent method, which doesn't get called in the described setup, because the AllenUpdater exclusively uses the IQueueingEventProcessor interface, which doesn't have nextEvent. Instead AllenUpdater manages a thread pool instead and HLTControlFlowMgr is updated to attach to a pre-existing pool.