Replace list based free/busy/requeue list with FIFO + ring

Cache friendliness of the list is pretty low. This has
provably lower overhead.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
Bump IO engine version number

We changed the layout of io_u, which isn't binary
compatible with pre-compiled external engines.

Signed-off-by: Jens Axboe <axboe@kernel.dk>