getLastCorrelationId()
Get the last correlation id that was used for making the REST API call.
Since this is an API with majority of request & response 
            processing happening asynchronously, it would be nice to know which callback handler
            is being processed for which request call. The API client/integration code can use
             correlationId as a way to identify the response for the
            corresponding request.  It's an optional argument to the API methods. If this value isn't passed,
            API will generate a new UUID every time before making the REST API call.
The integration code can use getLastCorrelationId() 
            method to find the last correlation id that was used for a given API request,
            on cases where the id wasn't passed to the API method.
The value of correlationId 
            will be the same for a given API request on their before and after 
            events.  The integration code can use correlationId 
            for tracking the event handlers specific to the events. 
Usage
Rupay.getLastCorrelationId()
Example
Rupay.getLastCorrelationId();
Arguments
None
Return Value
A String with the last correlation id that was used for making the REST API call.