Provide a way for the Tango clients to ignore events received after a synchronous call

During the subscription (or reconnection) to a Tango event, Tango will read the target attribute synchronously (executing read_attribute) and will execute the client event callback, providing an event containing the read synchronous value. In some use cases, the client is not interested in these events providing results from a synchronous call but is only interested in the events pushed by the device server. Tango should provide a way for the client to ignore/recognize these synchronous calls. This could be implemented as a new option in the subscribe_event methods and/or as a new member variable in the EventData class, a flag indicating whether this EventData object is the result of a synchronous call or not. It might be good to provide a way to identify easily as well the error events generated by the Tango client library and which have not been emitted by the device server.