add support for strict_type connections

Currently the strict_type=True option on the Meta class for a node is ignored and the connections end up having nullable values in the edge list. If you override the connection_class to set the strict_type on the connection, the SQLAlchemyConnectionField constructor fails because it does not unwrap the NonNull from the node before trying to set up the sort.

Ideally it should be possible to set the strict_type option on the connection via metadata on the node class.