Streamline thread_data data direction setting and checking

Streamline thread_data data direction setting and checking

Currently it's a mess of ->ddir, ->iomix and ->sequential. Add
a TD_DDIR_* for each of these, so we can store them as one value.

A prerequisite for the next parsing cleanup.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>