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

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