when committing a big transaction, online_alter_cache_log creates a cache
file. It wasn't properly closed, which was spotted by a memory leak from
my_register_filename. A temporary file also remained open.

Binlog wasn't affected by this, since it features its own file management.

A proper closing is calling close_cached_file. It deinits io_cache and
closes the underlying file. After closing, the file is expected to be
deleted automagically.