multi: don't expire timeouts at disonnect or done

The functions Curl_disconnect() and Curl_done() are both used within the
scope of a single request so they cannot be allowed to use
Curl_expire(... 0) to kill all timeouts as there are some timeouts that
are set before a request that are supposed to remain until the request
is done.

The timeouts are now instead cleared at curl_easy_cleanup() and when the
multi state machine changes a handle to the complete state.