Foreign key support

Foreign keys are now supported. However, not all the functionality is implemented, only the basics. Delete on cascade and so on are currently missing. More tests need to be implemented as well. There was a problem with MySQL, because in this database a index is required for the columns referencing. It may still be problematic in some cases, for example the MySQL documentation says CREATE INDEX after referential integrity is not supported, and currently LDBC does not check if the referenced table (parent table) already has an index. Also the DatabaseMetaData methods for foreign keys are not implemented / tested.