feat(p2p): ensure exhange can distinguish peers based on their pruning strategy #333
Description
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.