WIP: Use MPI_Testsome to control the activation of recv tasks

Please ignore, just to give an overview of the changes.

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• b69d0ee9 - Extend testsome tasks so we have one per subtype, should cut down on the numbers of unlocks

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• 2ce90a9c - Flags is only used in this context with testsome

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mentioned in issue #584

Doesn't add the hoped for improvements, so closing.

closed