Chore - cleanup internal routing

[shawn-zil]

Ideally, each message type should be queued in a separate queue. This allows each type to be treated equally, or prioritised - to mitigate any HOL-blocking issues.

A primitive mechanism is currently in-place:

1. Proposals are re-routed from a P2P-broadcast into the request queue; and
2. Sync messages are re-routed from a P2P-request into the broadcast queue.

If this form of coarse re-routing is sufficient, we may more correctly rename the two queues to reflect their actual usage.