[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.
usr/lib/PY_VERSION/*-packages/cubicweb/server/
usr/lib/PY_VERSION/*-packages/cubicweb/hooks/
usr/lib/PY_VERSION/*-packages/cubicweb/sobjects/
usr/lib/PY_VERSION/*-packages/cubicweb/schemas/
usr/share/cubicweb/migration/