Send end marker on GTP-U connection after a hand-over

[Andreas Schultz]

When ever a handover procedure changes the GTP-U path, a end marker GPT-U frames should be send on the old path to acknowledge the successful execution.

We need to:

1. send end marker frames to the old GTP-U peer when a remote tunnel endpoint changes
2. tear town GTP-U tunnels when we receive a end marker that matches a running tunnel

[Andreas Schultz]

this was implemented long ago with the shift to the UPF