feature/conn25: expire connector state

The connector struct holds a map of peer+transitIP -> destinationIP that
it uses for routing traffic. The client registers new entries in the map
over the peer API.

Stop the transitIPs map from growing indefinitely by expiring entries
after 1 hour.

Updates tailscale/corp#38261

Signed-off-by: Fran Bull <fran@tailscale.com>