Implement tree to track clients

[petehayes102]

• Instead of adding clients directly to a topic, add publishers and subscribers to respective trees.
• Implement a method for deduplicating the tree (i.e. if there are connections A => B and A => C, dedupe these into A => B + C).
• Implement a method for spawning any unspawned futures.

New sink + stream types will need to be implemented to allow each stream to have multiple downstreams, and each sink to have multiple drains. topic.rs can be the basis for both of these structures.