[entity,storage] remove hackish code from the bfss path
The ClientConnection/Connection duality brought some painful hacks
that are no longer needed.
Hence we can send entity._cw_update_attr_cache and ._cw_dont_cache_attribute
to the grave.
Related to #3837233
[connection] eliminate ClientConnection leftovers
* in the documentation
* in the docstrings
* in code notations (e.g. clt_cnx -> cnx)
Related to #3837233.
From now, there should be just one ClientConnection left in repoapi.py.