rpc/block: rework the way Block is JSONized

Our block.Block was JSONized in a bit different fashion than result.Block in
its NextConsensus and Index fields. It's not good for notifications because
third-party clients would probably expect to see the same format. Also, using
completely different Block representation is probably making our client a bit
weaker as this representation is harder to use with other neo-go components.

So use the same approach we took for Transactions and wrap block.Block which is
to be serialized in proper way.

Fix `Script` JSONization along the way, 3.0 node wraps it within `witnesses`.

pkg/rpc/server/server_test.go

@@ -384,9 +384,8 @@ var rpcTestCases = map[string][]rpcTestCase{
 				block, err := e.chain.GetBlock(e.chain.GetHeaderHash(3))
 				require.NoErrorf(t, err, "could not get block")
 
-				assert.Equal(t, block.Hash(), res.Hash)
-				for i := range res.Tx {
-					tx := res.Tx[i]
+				assert.Equal(t, block.Hash(), res.Hash())
+				for i, tx := range res.Transactions {
 					require.Equal(t, transaction.ContractType, tx.Type)
 
 					actualTx := block.Transactions[i]