Connection replaces ClientConnection everywhere.

Some notes:

* testlib: .client_cnx and .repo_cnx become aliases of .cnx (we might
  not want to tell people to update their tests again for just no real
  benefit, so we'll live with these aliases for a while)

* entity.as_rset must not be cached because we risk caching result
  sets having a Connection object as .req (helps unittest_breadcrumbs)

* entity._cw_dont_cache_attributes loses its repo/request special
  paths and only keeps its storage/bfss user (this helps
  unittest_wfobjs)

* moreover, entity.cw_instantiate and .cw_set stop overriding the
  attributes cache *after* the before_*_entity hooks have run, because
  there is no need to (it is now actually harmful to do it and
  unittest_hooks.test_html_tidy* tests remain green because of this)

* rset._build_entity sticks its .req onto the entity just fetched from
  the cache, because otherwise it might carry a _cw that is a
  Connection object where a Request is expected (helps
  unittest_views_actions)

* we get overall better cache usages (entity caches were split over
  Request + ClientConnection and Connection), hence the changes
  unittest_entity and unittest_wfobjs

* void the ecache when providing the cnx to a request object

  Having the entity cache pre-filled when we bind it to the request
  object hurts because these entities are bound to Connection objects,
  that lack e.g. `.form` or `.add_js` and crash the views subsystem.

  Thus, the unittest_testlib.test_error_raised test will are kept
  green.


Closes #3837233