Rename things in client_map.py

In the last rename, I saw nested sequences of dimension dictionaries being passed around and thought they were all `dim_data_per_rank` structures.  They are not; some of these are a de-duplicated zip of `dim_data_per_rank`.  Here I give those things their own name (`axis_dim_dicts`) and add some comments.

Also, rename `compactify_dicts` to `_dedup_dim_dicts`, and give `map_from_axis_dim_dicts` a leading underscore, since it shouldn't be part of the public interface.