This applies only for relays. Previous commit adds two new consensus
parameters that dictate how libevent is configured with DNS resolution.
And so, with a new consensus, we now look at those values in case they
ever change.

Without this, Exit relay would have to HUP or restart to apply any new
Exit DNS consensus parameters.

Related to #40312

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