Make ISF::CollectionMerger Algorithm Reentrant

John Derek Chapmanrequested to merge
jchapman/athena:ReentrantCollectionMerger_master into master

This merge request builds on the changes in !29909 (merged) to make the ISF::CollectionMerger AthAlgorithm into an AthReentrantAlgorithm.

The unit test has to be updated accordingly.

Edited by John Derek Chapman

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