Loss of Yggdrasil-capable peers when in mesh-only mode

[vartiovaara]

In some instances when connection gets interrupted for a long period of time in mesh-only mode, I2Pd can not regain connection at default reseed threshold. This seems to be a symptom of the yggdrasil routers getting dropped, leaving the rest as known. This can result in a situation where >900 routers are known, but none can do yggdrasil and reseeding is not triggered resulting in a dead router. This was only fixed by setting the reseeding threshold above that of the currently known routers and restarting.