PTO fires when unset for an abandoned path

[flub]

When a path is abandoned we sometimes still end up firing the PTO because it was set when packets were sent on the path. But some packets (e.g. PATH_NEW_CONNECTION_ID for the path) are then also removed for the path when the path is abandoned. Somehow this all results in the PTO no longer thinking it is set by the time it fires, which is an error.

This needs to be sorted out, we need to handle acknowledgements correctly.