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/rpc_test.go

@@ -66,7 +66,7 @@ func getResultBlock1() *result.Block {
 	if err != nil {
 		panic(err)
 	}
-	b := block.New(netmode.UnitTestNet, false)
+	b := block.New(false)
 	err = testserdes.DecodeBinary(binB, b)
 	if err != nil {
 		panic(err)