evdns: disable probing with EVDNS_BASE_DISABLE_WHEN_INACTIVE

When user install EVDNS_BASE_DISABLE_WHEN_INACTIVE flag for evdns base,
we must remove the timer that is used for probing, if current dns server
failed, otherwise it won't break the loop.
evdns: we don't need to add retry timer in transmit_to it will be done on fail
Merge pull request #129 from azat/disable-probing-with-disable-when-inactive-v2

evdns: disable probing with EVDNS_BASE_DISABLE_WHEN_INACTIVE