LES Peer Info

[rphmeier]

Closes #3900

May add fields for capabilities or buffer flow in the future, but for now it's the same as eth info, with the single exception that the head TD is always known.

Also adds a utility function for iterating all peers in the network.

[coveralls]

Coverage increased (+0.04%) to 84.868% when pulling fba4a49 on les-peer-info into 8852a05 on master.

[rphmeier]

I'm realizing now that this will also show peers who are in the "Handshake" state and aren't connected on eth, par, or les yet.
We can address this in two different ways: only show peer info for peers who are on at least one subprotocol, or we can have connected_peers only return peers who are fully connected. Alternatively, we can leave it as is, but it's doubtful that anything will want that info from the RPC call.

@arkpar any preference on how to handle it?

[arkpar]

"Handshake" as in subprotocol level handshake? e.g. Status exchange? I think it is fine to report unconfirmed/pending peers as long as there's a status field that allows to filter them out.