introduce peers registry on nodes

- TestPeerPromotionByTdOnBlock renamed and skipped for now test should pass iff
  if TD is updated based on an agreement
- senders register in AddBlock, flag records if they are coming from newblock
  message (and therefore advertise their TD with the block) or block message
  (TODO: latter are stored on the cache and updated by checkTD call;
  protocol should also call AddBlock on newblock messages by non-best peers)
- remove TD update from optional TD field in addBlock: this is no longer part of
  the eth protocol spec -> TODO: reflect in wiki
- only initialise peer map if at least two