ZeroNet as a platform for overlay networks.

Can ZeroNet be used as a library to create an overlay network of nodes interested in topic `foo/1.2.3` and let them exchange with arbitrary messages? Number of nodes: ~1M. Message size: < 1 KB.

For example, I want to make a distributed p2p version of a books catalogue. Ther catalogue has a well-known name, e.g. `books/1.2.3`. Nodes use this name to discover other nodes connected to this overlay network. Once connected, they download the list of book ids from a peer. If someone wants to add a book, they send its id to a few peers, they add it to the local storage and re-transmit to other peers. An important property of this system is that all the nodes are equal: there is no admin who has special rights. There is no special private key that has special rights in this catalogue.

API needed for this:
- `join("books/1.2.3")` finds a few peers in this overlay network
- `peers("books/1.2.3")` returns the list of online peers we're currently connected to
- `send("books/1.2.3", peerId, "howdy")` sends a message to the given peer
- `listen("books/1.2.3", callback)` gets messages sent by other peers with the `send()` API

Thoughts?

The issue does not identify files, tests, or an implementation entry point. First determine whether ZeroNet supports the proposed overlay-network model and API, then define the architecture and acceptance criteria for peer discovery, messaging, storage, and equal node permissions.

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