network: remove tier3_request_queue

[saketh-are]

Incoming state part requests are throttled by the view client actor, which drops excess requests to avoid overloading the resources of the node. This PR removes the additional layer of queueing on the network side of things. There is little benefit from queueing:

• State part requests are made at a regular rate over a long period of time and distributed well across nodes; there is no need for serving nodes to modulate spikes in demand
• If the steady flow of incoming requests is too high for the view client's throttling rate, queueing will change which requests are served but does not increase how many requests are served; there is no benefit in that

and a major downside:

• Queueing makes response time less predictable, forcing the requesting node to wait longer before deciding that a request was dropped/failed/lost

The queue served a secondary purpose, which is that it allowed PeerActors receiving state part requests to hand them off to the PeerManagerActor. Instead we set up an adapter so that the requests can be passed as actix messages.