Make Local Onion Persistent Between Restarts

Unless mistaken, in `TorManager.py`, method `addOnion`, the private keys are never saved anywhere and never re-used preventing the onion addresses from being persistent between ZeroNet restarts.

Since libzeronet's primary concern is performance for resilient super-peers (eligible for eventual inclusion in the bootstrapable peer list in `Config.py`), this issue is libzeronet-related and a solution for it might be included in my libzeronet-backed port of the Python stack.