Get description post-processing does too many IO operations

[willdealtry]

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The post-processing for get_description_batch does a lot of individual IO operations, which kind of defeats the point of doing batch operations at all. We should ensure that we return the appropriate data from the initial C++ calls and any post-processing is in-memory only: https://github.com/man-group/ArcticDB/blob/master/python/arcticdb/version_store/_store.py#L2494