mount/fuse: enable fuse real async dio when available

fuse has support for optimized async. direct I/O handling via the
FUSE_ASYNC_DIO init flag. Enable FUSE_ASYNC_DIO when advertised
by fuse.

performance/write-behind: fix dio hang

Also fix a hang observed during aio-stress testing due to conflicting
request handling in write-behind. Overlapping requests are skipped
in pick_winds and may never continue when the conflicting write in
progress returns. Add a wb_process_queue() call after a non-wb request
completes to keep the queue moving.

BUG: 963258
Change-Id: Ifba6e8aba7a7790b288a32067706b75f263105d4
Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-on: http://review.gluster.org/5014
Reviewed-by: Anand Avati <avati@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>