[API] bi-directional automated messaging to query peers

[daviddahl]

use case: a new peer subscribes to the IPFS_SOCIAL_PROOF topic and should immediately broadcast its identity information: pubkey, handle, etc.

see: https://github.com/IBM/ipfs-social-proof/blob/master/src/index.js#L93

Peers should update the peer list.

It may also be handy to keep track of peers in level-db: peerHash, firstSeen, lastSeen, currentHandle, etc

[daviddahl]

Perhaps we log the peer joined event, but do not display the peer in the peers list until we have the initial broadcast?