sheep/md: change process of for_each_object_in_wd() to multi-threads

In our test environment, we upload 6TB data and kill one node, then the sheep
daemons on each server will try to rename files from 'data' to '.stale', but
the rename operations cost almost half an hour. And in this half an hour, the
whole cluster can't be read or write.

To accelerate the speed of renames, we change for_each_object_in_wd() to
multi-threads, which is one thread for one disk.

Signed-off-by: Robin Dong <sanbai@taobao.com>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>