split the references into 2 tables to allow using defered FKs in the future

Upgrade steps:
Upgrade jackalope/jackalope-doctrine-dbal
Run ``app/console doctrine:phpcr:init:dbal --dump-sql``
Copy and execute all tables, indexes etc related to ``phpcr_nodes_references`` and ``phpcr_nodes_weakreferences``.

Run the following SQL statements:
INSERT INTO phpcr_nodes_references ( source_id, source_property_name, target_id )
SELECT source_id, source_property_name, target_id
FROM phpcr_nodes_foreignkeys WHERE type = 9;
INSERT INTO phpcr_nodes_weakreferences ( source_id, source_property_name, target_id )
SELECT source_id, source_property_name, target_id
FROM phpcr_nodes_foreignkeys WHERE type = 10;

Finally clean up the old table:
DROP TABLE phpcr_nodes_foreignkeys;

In the future we can now implement the deferred FK support:
Doctrine DBAL supports this for PostgreSQL, it should be possible to add this to SQLite as well:
https://github.com/doctrine/dbal/pull/220

http://www.postgresql.org/docs/9.2/static/sql-set-constraints.html
http://www.sqlite.org/foreignkeys.html#fk_deferred