Connection clean up

We need to clean up the connection file descriptor and the gss context.

* tidc_fwd_request is the wrong place to free the TID_REQ because it does not allocate it

* tid_req_dup needs to use talloc; duplicated requests are freed by
  the original request

* tidc_send_message frees its request

* The request destructor closes the connection.

* Keep track of whether a request is duplicated; only free the connection and gss context in the original request.