If refresh stale RRset times out, start stale-refresh-time

If refresh stale RRset times out, start stale-refresh-time

The previous commit failed some tests because we expect that if a
fetch fails and we have stale candidates in cache, the
stale-refresh-time window is started. This means that if we hit a stale
entry in cache and answering stale data is allowed, we don't bother
resolving it again for as long we are within the stale-refresh-time
window.

This is useful for two reasons:
- If we failed to fetch the RRset that we are looking for, we are not
  hammering the authoritative servers.

- Successor clients don't need to wait for stale-answer-client-timeout
  to get their DNS response, only the first one to query will take
  the latency penalty.

The latter is not useful when stale-answer-client-timeout is 0 though.

So this exception code only to make sure we don't try to refresh the
RRset again if it failed to do so recently.