sheep: fix sheep sometimes cannot be killed successfully

I am sure this is a bug because the variable nr_outstanding_reqs
does not reset to zero sometimes after client cancelling the
request. Once nr_outstanding_reqs is not zero, sheep process
never be killed successfully, neither using kill <pid> nor
using dog node kill command.

But I am not sure whether the bug is fixed perfectly because
I am not familiar with the sheepdog networking logic. I have to
add some error messages to every doutful statements.

The result is, I caught one of them. So, I call the function
clear_client_info in that place. It seems every thing is fine
after the modification.

Does anyone help to investigate and fix it?

Signed-off-by: Ruoyu <liangry@ucweb.com>
Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>