[SAFS]: global_cached_io respects the I/O order given by apps.
[SAFS]: global_cached_io respects the I/O order given by apps.

It will still try its best to merge I/O requests before pushing the reqs
to remote I/O.

silky/FlashGraph,icoming/FlashX,silky/FlashGraph,icoming/FlashGraph,icoming/FlashX,icoming/FlashGraph,icoming/FlashX,flashxio/FlashX,flashxio/FlashX,zheng-da/FlashX,flashxio/FlashX,icoming/FlashX,icoming/FlashGraph,icoming/FlashGraph,silky/FlashGraph,flashxio/FlashX,zheng-da/FlashX,silky/FlashGraph,icoming/FlashGraph,silky/FlashGraph,zheng-da/FlashX,flashxio/FlashX,icoming/FlashX,flashxio/FlashX,zheng-da/FlashX,zheng-da/FlashX