Fix ThinningSvc to retrieve OutputStream from the correct sequence.

This may help with ATLASRECTS-4961

Thanks @amete , @ssnyder , @jcatmore , @christos , @mhodgkin , @goetz

This merge request affects 1 package:

• Control/AthenaServices

Adding @ssnyder ,@rbianchi as watchers

added Core master review-pending-level-1 labels

CI Result SUCCESS

	Athena	AthSimulation
externals
cmake
make
required tests
optional tests

Full details available at NICOS MR-23872-2019-06-01-00-50
Athena: number of compilation errors 0, warnings 1
AthSimulation: number of compilation errors 0, warnings 0
For experts only: Jenkins output [CI-MERGE-REQUEST 39297]

added review-approved label and removed review-pending-level-1 label

merged

mentioned in commit 015f2a2a

added sweep:ignore label