Add new I/O scheduler FIFO.

Does not perform any merging or reordering -- pure FCFS.

Signed-off-by: Aaron Carroll <aaronc@gelato.unsw.edu.au>
noop-iosched: perform full merging

Make noop perform full back- and front-merging and proper coalescing.
Also changes dispatching to back-insertion (rather than sorted) onto
the dispatch queue.

Signed-off-by: Aaron Carroll <aaronc@gelato.unsw.edu.au>
Forward porting some FIFO/No-Op changes