block: drop Network from the Header

It's not network-tied any more, network is only needed to
sign/verify. Unfortunately we still have to keep network in consensus data
structures because of dbft library interface.

pkg/rpc/client/wsclient.go

@@ -139,7 +139,7 @@ readloop:
 			var val interface{}
 			switch event {
 			case response.BlockEventID:
-				val = block.New(c.GetNetwork(), c.StateRootInHeader())
+				val = block.New(c.StateRootInHeader())
 			case response.TransactionEventID:
 				val = &transaction.Transaction{}
 			case response.NotificationEventID: