Add output ByteStream to output sequence ATLASRECTS-6130

The multiple stream manager was adding configured ByteStream output streams in the old Gaudi style to "theApp". In multithreaded transforms this means they no longer appear in the AthOutSeq, and thus no output is written. This commit fixes that.

Database/AthenaPOOL/OutputStreamAthenaPool/python/MultipleStreamManager.py

-# Copyright (C) 2002-2020 CERN for the benefit of the ATLAS collaboration
+# Copyright (C) 2002-2021 CERN for the benefit of the ATLAS collaboration
 
 ########################################################
 ##  AugmentedStreams & MultipleStreamManager classes  ##
@@ -284,12 +284,14 @@ class AugmentedByteStream( AugmentedStreamBase ):
         self.bsCopyTool.ByteStreamOutputSvc=self.bsOutputSvc
         self.bsCopyTool.ByteStreamInputSvc=svcMgr.ByteStreamInputSvc
 
-        # create AthenaOutputStream for BS Copy and add it to topSequence
+        # create AthenaOutputStream for BS Copy
         from AthenaServices.AthenaServicesConf import AthenaOutputStream
         self.Stream = AthenaOutputStream( StreamName, WritingTool=self.bsCopyTool )
 
-        #topSequence += self.Stream #<-- coherent with asAlg=False in OutputStreamAthenaPool.py
-        theApp.addOutputStream( self.Stream )
+        # add the stream to the output sequence
+        from AthenaCommon.AlgSequence import AthSequencer
+        outSequence = AthSequencer("AthOutSeq")
+        outSequence += self.Stream
         return
 
     def SetOutputFileName(self, name):