Add row_factory as connection attribute and connect argument

Add row_factory as connection attribute and connect argument

When passing 'row_factory' to connect(), respective attribute will be
set on the connection instance. This will be used as default at cursor
creation and can be overridden with conn.cursor(row_factory=...) or
conn.execute(row_factory=...).

We use a '_null_row_factory' marker to handle None-value passed to
.cursor() or .execute() for disabling the default row factory.