by long running transaction

On Windows opened files can't be deleted. There was a special
upgraded lock mode (TL_WRITE instead of TL_WRITE_ALLOW_READ) 
in ALTER TABLE to make sure nobody has the table opened
when deleting the old table in ALTER TABLE. This special mode
was causing ALTER TABLE to hang waiting on a lock inside InnoDB.
This special lock is no longer necessary as the server is 
closing the tables it needs to delete in ALTER TABLE.
Fixed by removing the special lock.
Note that this also reverses the fix for bug 17264 that deals with
another consequence of this special lock mode being used.