Fix a bug in global_cached_io for pending reqs on pages.
Fix a bug in global_cached_io for pending reqs on pages.

In the current model, all requests must be completed in the thread where
they are issued. Therefore, we need to redistribute the pending
requests.

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