feat(p2p): ensure exhange can distinguish peers based on their pruning strategy

Currently, the exchange protocol assumes every peer has the whole history of headers. As pruning lands, this assumption will no longer hold. However, the protocol should still strive to find all the headers in the p2p network, such that an exchange client can request any header in the past without needing to store them locally.

The simplest solution is to assume we have at least one connection to a node retaining the whole history. It can be either manually configured, e.g., TrustedPeer, or discovered over DHT, or probed out of immediate connections.

Start by tracing the exchange protocol and how peers are configured or discovered, including TrustedPeer, DHT discovery, and probing immediate connections. Define how a client identifies peers by pruning strategy and verify that requests for any historical header can be routed to a peer retaining the needed history.

Written by the indexing model from the issue text.