Backout "[entity,storage] remove hackish code from the bfss path
The merge of the repo-side and web-side entity caches doesn't actually work,
because entities have a _cw attribute which is sometimes a connection and
sometimes a web request, and whose nature changes not-really-predictably
whenever ResultSet.get_entity() is called and brings it out of the cache.
This reverts commit 59a79300f213.