[repoapi] fold ClientConnection into Connection 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

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