fixed the bp3 serializer bug in AggregateCollectiveMetadataIndices function

Podhorszki, Norbertrequested to merge
github/fork/lwan86/bp3-bug-fix into master

Created by: lwan86

This is a performance bug. Before being fixed, the complexity of generating the collective metadata indices is n^2, where n is the number of writers. Now the complexity is n.

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