Only start stale refresh window when resuming

Only start stale refresh window when resuming

If we did not attempt a fetch due to fetch-limits, we should not start
the stale-refresh-time window.

Introduce a new flag DNS_DBFIND_STALESTART to differentiate between
a resolver failure and unexpected error. If we are resuming, this
indicates a resolver failure, then start the stale-refresh-time window,
otherwise don't start the stale-refresh-time window, but still fall
back to using stale data.

(This commit also wraps some docstrings to 80 characters width)