p2p: persistent peers list

[oleganza]

Currently peers have to manually seeded through an API. We need:

1. Support for config to specify a list of seed peers.
2. Store recently used peers in a separate file, so we can reconnect faster after restart.
3. Store banned/deprioritized peers also, so we cannot be tricked to forget misbehaving nodes when crashed by some unpatched DoS vector.