Expose list of peers with too old code versions

[sesam]

Maybe grinexplorer could have an output with the most up to date list of seemingly unwanted peers.

This could be formatted like a dynamic "getting started with grin" kind of living documentation, that gets autogenerated from the last known grin network situation.

By formatting it as one peer per row, curl -X POST http://localhost:13413/v1/peers/$IP:$PORT/ban, anyone having trouble syncing up or who want to avoid a lot of gossiping with outdated peers could copy&paste the list to their local node. This is meant as a help in messy situations when a breaking update has been made and lots of nodes are not yet updated.

Note that grin's ban API endpoint is currently "forever" but will become time limited, but until that is done, it might also be a good idea to keep a note about cleaning out the ban list manually by removing .grin/peers as soon as you've got your node synced up.