Support for MySQL's MyISAM and other non-InnoDB engines in SchemaTool

MySQL allows foreign keys only between tables using the InnoDB engine (MyISAM, memory and other engines don't support foreign keys).
This PR delay foreign keys generation after the hydration of all `Table` objects. It enables DBAL to check tables engines before adding foreign keys.

This PR is related to https://github.com/doctrine/dbal/pull/437