Prevent livelocks in hoodselector on mesh routers

As discussed at the meeting on 9. Jan. the hoodselector should select the best quality neighbor which was not tested before. The file of tested bssids can be deleted when there are no more ffnw-networks available, so the process starts over.

RAM is not a problem here because it's already unusual to have 3 ffnw-networks at one location an it's not big data all over.