Fix the spin bit becoming immediately disabled for servers

We were disabling it every time we updated the remote CID, and we like
to update the remote CID ASAP for servers to enable client-side
stateless resets. Instead, we should disable it only when switching
remote CIDs due to a migration.

quinn-proto/src/connection/mod.rs

@@ -2586,6 +2586,7 @@ impl Connection {
             self.migrate(now, remote);
             // Break linkability, if possible
             let _ = self.update_rem_cid();
+            self.spin = false;
         }
 
         Ok(())
@@ -2643,9 +2644,6 @@ impl Connection {
                 reset_token,
             ));
         self.peer_params.stateless_reset_token = Some(reset_token);
-
-        // Reduce linkability
-        self.spin = false;
     }
 
     /// Issue an initial set of connection IDs to the peer