fix: when running multiple instances, ORM metadata cause crash

[pekasen]

InvalidRequestError: Table 'node' is already defined for this MetaData instance.  Specify 'extend_existing=True' to redefine options and columns on an existing Table object.

The Metadate object should be insulated and used only one instance.

[pekasen]

This is caused by the ORM-classes being defined on the module level, i.e. the objects for all Spider-instances inherit from the same base-class, thus, re-using table names is disallowd.

Solution would be to wrap the DB-handling inside a class.