Subscribe to the "mailbox topics" of close peers

[lyulka]

Background

Right now, the only peer which subscribes to a particular "mailbox topic" (the topic associated with a particular Public Address) is the replica which is identified by that Public Address.

This means that unicast messages are never sent eagerly and through full-message pairings. Instead, for a unicast message to arrive at its recipient:

1. Its metadata record needs to be gossiped to the recipient.
2. The recipient then needs to state that it wants the message ("IWANT").
3. The recipient then needs to wait until the IWANT arrives at a peer which has the full message: either the sending peer, or a peer which was lucky enough to have been gossiped the message in a heartbeat (by default, 1 second).
4. Finally, the recipient needs to wait for the message to finally arrive.

This makes the sending of unicast messages slow.

Proposed solution

Peers should subscribe to the individual topics of other peers that are close to them. This greatly increases the likelihood that a sending peer which is sending a unicast message has a full-message peering for the mailbox topic of the recipient, and therefore will send the message immediately.

[lyulka]

Moved to parallelchain-io/parallelchain-protocol#12.