Add a way to whitelist specific block numbers and hashes to choose our fast sync peers based on these, similar in functionality to geth's `--whitelist value Comma separated block number-to-hash mappings to enforce (<number>=<hash>)`.
We should use the PeerValidator interface for this (see the class).
CLI flag to specific a block number and its associated hash to whitelist it
Fast sync check for this flag when finding peers to sync from
Unit and acceptance tests