This MR addresses some short comings of I/O when running AthenaMT and mainly writing to multiple streams:
Multiple stream writing has to be serial (for now) as it modifies the TransientAddress of output objects. Therefore a mutex is introduced into the ItemList interface to allow the AthenaOutputStream serialization of streaming.
A warning for missing AttributeList is suppressed unless it in the EventStore
Use of ROOT Implicit Multithreading can be disabled via PoolSvc