Fix tracing for chunk delivery

[nonsense]

In the last few weeks, we improved tracing with respect to retrieve requests. Currently the request part of the trace is mostly meaningful:

1. However the delivery part is missing. The spans defined in Peer.Deliver (swarm/network/stream/delivery.go) are broken. We should link them with the respective request part of the trace.

2. Delivery spans also start via the code in Peer.SendPriority, but there is no code that actually calls Finish for them.

Because of 1 and 2, we see when a chunk is retrieved locally on a given node (localstore.get span), but we have no idea when it is actually delivered to the requesting node.

cc @nolash

[nonsense]

After replacing SendPriority with Send, we can now trace a request from requestee node, back to requestor. It looks like we have a bug in the fetcher, because it is clear that a chunk is delivered, but the fetcher still requests it again from other peers: