rename server.session.transaction into server.session.connection
The ongoing rework of the API to access the database include the splitting of
Session in two objects:
The ``Session`` object will only hold credential (user) and global session data.
A new object will publicly emerge to handle a database access. This object is
named ``Connection`` since that the way database accessors with the same property
are named in other system. So we renamed the ``Transaction`` object into
``Connection``. The ``Transaction`` object have already grown in the direction
of something directly usable by generic code, but the name ``Transaction`` is
ill suited as such object can be used for multiple database transaction in a
row.
Related to #2503918