IOVDbMetaDataTools: Clean up overrideIOV.
IOVDbMetaDataTool::overrideIOV doesn't actually need to be declared as not thread-safe. Instead of copying the input collection, we can modify the IOVs in place.
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IOVDbMetaDataTool::overrideIOV doesn't actually need to be declared as not thread-safe. Instead of copying the input collection, we can modify the IOVs in place.