Fix the statistic counter underflow in ns_client_t

Fix the statistic counter underflow in ns_client_t

In case of normal fetch, the .recursionquota is attached and
ns_statscounter_recursclients is incremented when the fetch is created.  Then
the .recursionquota is detached and the counter decremented in the
fetch_callback().

In case of prefetch or rpzfetch, the quota is attached, but the counter is not
incremented.  When we reach the soft-quota, the function returns early but don't
detach from the quota, and it gets destroyed during the ns_client_endrequest(),
so no memory was leaked.

But because the ns_statscounter_recursclients is only incremented during the
normal fetch the counter would be incorrectly decremented on two occassions:

1) When we reached the softquota, because the quota was not properly detached
2) When the prefetch or rpzfetch was cancelled mid-flight and the callback
   function was never called.