Flush the DDL log on FLUSH LOGS

When ALTER TABLE is executed, some (rollback?) metadata is written to a DDL log file in the datadir. This file cannot be rotated on-the-fly - only a restart of the mysqld allows it to be deleted. On systems which make a heavy use of ALTER TABLE the log file easily reaches gigabytes within days. The DDL log was first seen on Oracle MySQL 5.7.

This request asks for a change in behaviour so that the FLUSH LOGS statement will apply to the DDL log also in the same was as it does to binary logs et al.