On-demand pubsub

Currently, pubsub will startup when initialized. Unfortunately, this means:

• Running three goroutines per peer (at least?).
• Processing subscriptions from peers.

This is true even if pubsub is enabled but not in-use.

In order to enable pubsub by default in go-ipfs, we need to find some way for pubsub to not take up a bunch of resources when not in-use.

The MVP solution is on-demand startup. We can start pubsub on-demand and stop after some idle timeout after the last subscription closes. This should be fairly simple to implement and will make it possible for us to turn pubsub on by default without significantly and increasing resource usage for nodes that aren't even using pubsub.

The ideal solution is idle peer detection. We don't really need to keep a stream/goroutine open per peer and could instead close streams to peers to which we have not spoken in a while. At the moment this will make the peer think we're dead so we may need a protocol change to implement this correctly.

Start by tracing pubsub initialization, peer goroutines, and subscription lifecycle in the Go implementation. Define the on-demand startup and idle-timeout behavior, then verify that pubsub does not consume those resources while unused and stops after the last subscription closes; leave ideal idle-peer detection and protocol changes out of scope.