rpc/block: rework the way Block is JSONized

Our block.Block was JSONized in a bit different fashion than result.Block in
its Nonce and NextConsensus 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 in result 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.Base which is
to be serialized in proper way.

pkg/rpc/server/server_test.go

@@ -336,7 +336,7 @@ var rpcTestCases = map[string][]rpcTestCase{
 				block, err := e.chain.GetBlock(e.chain.GetHeaderHash(2))
 				require.NoErrorf(t, err, "could not get block")
 
-				assert.Equal(t, block.Hash(), res.Hash)
+				assert.Equal(t, block.Hash(), res.Hash())
 				for i := range res.Tx {
 					tx := res.Tx[i]
 					require.Equal(t, transaction.MinerType, tx.Transaction.Type)