[session] remove session-related bw compat code Related to #3933480.

[session/transaction] removed unused transaction class Related to #3933480.

[transactions] cleanup after the dbapi removal * end req vs cnx dichotomy * Transaction takes a cnx at __init__ time * a couple of super calls are fixed Related to #3933480.

[session/cnx] remove `session_handled` logic Related to #3837233 .

[web/sessions] reduce repoapi usage where possible Using session.new_cnx() should be the new preferred way. Related to #3837233.

[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.

[session] all cnx._session become cnx.session The _session belonged to ClientConnection, which is just gone. Related to #3837233.

[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

[migractions] remove any session related leftovers Also, the migration handler really wants a "repo" session anyway. This highlights a bug in unittest_wfobjs.test_duplicated_transition where an old connection is reused. It probably used to work because it was made of session + set_cnxset magic dust. Related to #3933480.