Here we are modifying the port mapping logic so that existing port
mappings will only be removed when they were previously created by geth.
The AddAnyPortMapping functionality has been adapted to work consistently
between the IGDv1 and IGDv2 backends.
This changes the port mapping procedure such that, when the requested port is unavailable
an alternative port suggested by the router is used instead.
We now also repeatedly request the external IP from the router in order to catch any IP changes.
Co-authored-by: Felix Lange <fjl@twurst.com>
