openflow: Change DeferredSender shutdown

This hopefully shapes up DeferredSender's startup and shutdown
behavior.  For startup, we now initialize on of_01 launch
rather than module load.  For shutdown, we no longer run as a
daemon thread (AND wake often); we now explicitly wake it up
on GoingDownEvent.  This will hopefully solve a race condition
where slow boxes sometimes got an exception when shutting down.
tests: Add a byte order test

There are actually other cases we should be testing too,
but at least this is a start.
libopenflow: Fix decoding of OFPST_TABLE requests

OFPST_TABLE, like OFPST_DESC, has an empty request body.  The
latter was already addressed, but the former wasn't.  They
should now be handled the same way.

Currently untested.