lazy-pages: handle_remaining_pages: enable asynchronous reads

Until now once we've started to fetch an iovec we've been waiting until
it's completely copied before returning to event processing loop. Now we
can have several request for the remote pages in flight.

Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
Acked-by: Pavel Emelyanov <xemul@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>