block/transaction: add network magic into the hash

We make it explicit in the appropriate Block/Transaction structures, not via a
singleton as C# node does. I think this approach has a bit more potential and
allows better packages reuse for different purposes.

pkg/rpc/server/server_helper_test.go

@@ -50,7 +50,7 @@ func getTestBlocks(t *testing.T) []*block.Block {
 	blocks := make([]*block.Block, 0, int(nBlocks))
 	for i := 0; i < int(nBlocks); i++ {
 		_ = br.ReadU32LE()
-		b := &block.Block{}
+		b := block.New(netmode.UnitTestNet)
 		b.DecodeBinary(br)
 		require.Nil(t, br.Err)
 		blocks = append(blocks, b)