Prefer low-latency peers

[cloudhead]

Currently, peer latency is measured in the PingManager, but not used.

• We should keep track of latencies, persist them (either in the client::peer::Cache or some where else.
• Use it to decide which peers to connect to
• Use it to decide which peers to download headers/blocks from

Some notes:

• We should generally not compromise geo-diveristy when looking for low-latency peers, but having a few low-latency peers is desired
• Low-latency peer selection can either be via AddressManager::sample being biased towards low-latency peers, or by simply calling sample 2-3 times and picking the peer with the lowest latency.