Determine which peers to communicate with

[germsvel]

We currently have part of the ethereum wire protocol (sub-protocol) v62 implemented in lib/ex_wire/sync.ex. Thus far, that module can send GetBlockHeaders, GetBlockBodies, and receives BlockHeaders and BlockBodies. But when we need to send out a message, we ask the supervisor of all the peers (ExWire.PeerSupervisor) to send the message to all peers.

I do know if that is the best way to handle requesting things like GetBlockHeaders, or if we should be targeting a particular peer (if we have received a message from it), or if we should be doing it round robin or something like that.

[InoMurko]

I think this is related to #639.