Support per-topic TCP connections

Can we optionally support dedicated TCP connections per-topic?

This would make it easier to implement traffic prioritization / rate-limiting based on topic.

Start by examining how go-libp2p-pubsub currently manages TCP connections and topic traffic. Define the scope and design for optionally dedicating connections per topic, including how traffic prioritization and rate limiting would use it; the issue provides no specific files or tests to begin with.

Written by the indexing model from the issue text.