AthenaPoolCnvSvc: Initialize members in header

Walter Lamplrequested to merge
wlampl/athena:AthenaPoolCnvSvcProperties into master

AthenaPoolCnvSvc: Initialize members in header except of ToolHandleArray OutputStreamingTool because of gaudi/Gaudi#122 (closed)

The PoolAttributes property uses now set-semantics which should resolve ATLASSIM-4520.

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