[Sheepdog] [PATCH v2] refactor handling io request by ops.c framework

[Liu Yuan]

Since we already have a low level framework to handle requests, let's use
it to handle io requests too.

Thanks,
Yuan

v2 changes:
 - remove read_copy_one
 - rename do_local_rw to do_local_io
 - remove 'extern' in header

 above suggested by Christoph Hellwig, thanks!

Liu Yuan (3):
      sheep: unify cow object and regular object writing path
      sheep: refactor local and io request handling
      sheep: use do_process_work() to handle io request