Add TODOs on uncertain points about time_since_last_traffic

This edge-case was there even before the migration of
595fe1ab88, but now it's more explicit and ought to be
revisited.

crates/tor-circmgr/src/timeouts/pareto.rs

@@ -584,6 +584,7 @@ impl super::TimeoutEstimator for ParetoTimeoutEstimator {
             if let Some(last_traffic) = tor_proto::time_since_last_incoming_traffic() {
                 last_traffic < delay
             } else {
+                // TODO: Is this the correct behavior in this case?
                 true
             };
 

crates/tor-guardmgr/src/lib.rs

@@ -477,6 +477,7 @@ impl<R: Runtime> GuardMgr<R> {
                     .all_primary_guards_are_unreachable()
                     && duration >= inner.params.internet_down_timeout
             } else {
+                // TODO: Is this the correct behavior in this case?
                 false
             };