relay: Overload state on DNS timeout is now X% over Y secs

With this commit, we will only report a general overload state if we've seen more than X% of DNS timeout errors over Y seconds. Previous behavior was to report when a single timeout occured which is really too small of a threshold.

The value X is a consensus parameters called "overload_dns_timeout_scale_percent" which is a scaled percentage (factor of 1000) so we can represent decimal points for X like 0.5% for instance. Its default is 1000 which ends up being 1%.

The value Y is a consensus parameters called "overload_dns_timeout_period_secs" which is the time period for which will gather DNS errors and once over, we assess if that X% has been reached ultimately triggering a general overload signal.

Closes #40490 (closed) Closes #40491 (closed)

Signed-off-by: David Goulet dgoulet@torproject.org