Peer eviction strategy

[achingbrain]

JS does not favor long lived peers.

1. We need to implement ping. When a useful peer becomes known, we add it to the routing table if there is room. If there is not room, we should NOT evict peers without pinging/dialling them.
2. Useful peers are those that have successfully responded to a query.

[achingbrain]

1. is implemented by feat: ping old DHT peers before eviction #229

[SgtPooki]

@achingbrain looks like we can close this?

[achingbrain]

I don't think the second part has been done yet - adding some metadata to the routing table that says a peer has responded to a query.