JsonRpcServiceWrapper: use Jackson to map to custom types (after method is identified)

[msgilligan]

Because the JsonRpcRequest is deserialized before we identify the method name, the parameters (of type Object) are deserialized to generic Java types. If we delay the deserialization (probably by using the JsonNode type as an intermediate representation) we should be able to use the Method object for the method to deserialize to specific types (e.g. Coin, etc.)