Expiring peerstore data

[achingbrain]

The peerstore needs to be smarter about expiring old data and evicting older peers.

• Multiaddrs should expire after a timeout - go-libp2p uses one hour
• Peers without multiaddrs should be evicted after a period of time, perhaps six hours
• The multiaddr removal/eviction could be done while calling .all/.get on the peerstore