Last Value Cache

Nobody has claimed this yet.

In the search for a solution to Orbits problem of how to know about the current state of a chat when joining, we had the idea of adding a last value cache to the floodsub implementation.

> This [last value caching] solves the problem of how a new subscriber catches up when it joins the network. The theory is that publishers get notified when a new subscriber joins and subscribes to some specific topics. The publisher can then rebroadcast the last message for those topics.
>
> From http://zguide.zeromq.org/php:chapter5#Last-Value-Caching

I have an experimental implementation for js up here: https://github.com/libp2p/js-libp2p-floodsub/pull/32 which caches the last known message for all known topics currently. This definitely needs some refinements, to avoid things like unlimited memory usage.

My first tests with orbit showed that this does solve the issue of message loading on join, without any changes to the codebase.

Start by reviewing the experimental JavaScript implementation in libp2p/js-libp2p-floodsub PR #32 and the floodsub behavior described here. Determine how publishers learn about new subscribers and how the last message is cached per topic. Done means new subscribers receive the latest topic message without unbounded memory usage.

Written by the indexing model from the issue text.