* Fix race condition and ensure correct message ordering in
connection establishment
Reorder operations in OpenConn to register the connection before
waiting for peer availability. This ensures:
- Connection is ready to receive messages before peer subscription
completes
- Transport messages and onconnected events maintain proper ordering
- No messages are lost during the connection establishment window
- Concurrent OpenConn calls cannot create duplicate connections
If peer availability check fails, the pre-registered connection is
properly cleaned up.
* Handle service shutdown during relay connection initialization
Ensure relay connections are properly cleaned up when the service is not running by verifying `serviceIsRunning` and removing stale entries from `c.conns` to prevent unintended behaviors.
- Move `util/grpc` and `util/net` to `client` so `internal` packages can be accessed
- Add methods to return the next best interface after the NetBird interface.
- Use `IP_UNICAST_IF` sock opt to force the outgoing interface for the NetBird `net.Dialer` and `net.ListenerConfig` to avoid routing loops. The interface is picked by the new route lookup method.
- Some refactoring to avoid import cycles
- Old behavior is available through `NB_USE_LEGACY_ROUTING=true` env var
