evdns: fail ns after we are failing/retrasmitting request

evdns: fail ns after we are failing/retrasmitting request

In case we are failing request (evdns_request_timeout_callback()), we
delete timeout_event in request_finished(), while just before calling
request_finished() (for failing request) there was a call to
nameserver_failed(), that add event for timeout_event, IOW we must fail
ns after request because otherwise we will not have timeout_event
actived, and we will waiting forever.

Before this patch the dns/retry_disable_when_inactive will wait forever,
after - OK.