feat(snownet): minimize delay when roaming (#4246)

Currently, we need to wait for the timeout of the current candidate pair
during `reconnect` before we nominate a new one. To speed this up, we
can preemptively invalidate candidates we have previously discovered via
our `Allocation`s, i.e. relay candidates and srflx candidates.

rust/connlib/snownet/src/allocation.rs

@@ -756,8 +756,9 @@ fn update_candidate(
 ) {
     match (maybe_new, &maybe_current) {
         (Some(new), Some(current)) if &new != current => {
-            *maybe_current = Some(new.clone());
-            events.push_back(CandidateEvent::New(new));
+            events.push_back(CandidateEvent::New(new.clone()));
+            events.push_back(CandidateEvent::Invalid(current.clone()));
+            *maybe_current = Some(new);
         }
         (Some(new), None) => {
             *maybe_current = Some(new.clone());

rust/connlib/snownet/src/node.rs

@@ -731,6 +731,7 @@ where
 
         let mut agent = IceAgent::new();
         agent.set_controlling(true);
+        agent.set_max_candidate_pairs(300);
 
         let session_key = Secret::new(random());
         let ice_creds = agent.local_credentials();